Implement ntopng plugins

Plugins are a convenient way to group together related lua scripts.
Their primary use case is to group user scripts and their alert/status
definition.
The builtin ntopng user scripts and definitions are now
packed into plugins directories. In future, we will support loading of
user created plugins.
Plugins are loaded at startup into some runtime directories and then
used. Other changes provided by this commit include:

- Add sample flow logger plugin
- Initial support for system user scripts
- Rename edge to threshold
- Migrate system probes to user scripts/plugins
- Migrate scripts to more explicit alerts_api.checkThresholdAlert api

scripts/plugins/rtt/user_scripts/system/rtt.lua

@@ -0,0 +1,162 @@
+--
+-- (C) 2019 - ntop.org
+--
+
+local alerts_api = require("alerts_api")
+local alert_consts = require("alert_consts")
+local user_scripts = require("user_scripts")
+local rtt_utils = require("rtt_utils")
+local ts_utils = require("ts_utils_core")
+
+-- Enable do_trace messages
+local do_trace = false
+
+local script = {
+  -- This module is enabled by default
+  default_enabled = true,
+
+  -- Exclude this script in Windows
+  windows_exclude = true,
+
+  -- No default configuration is provided
+  default_value = {},
+
+  -- See below
+  hooks = {},
+}
+
+-- #################################################################
+
+local function pingIssuesType(value, threshold, ip)
+  return({
+    alert_type = alert_consts.alert_types.alert_ping_issues,
+    alert_severity = alert_consts.alert_severities.warning,
+    alert_granularity = alert_consts.alerts_granularities.min,
+    alert_type_params = {
+      value = value, threshold = threshold, ip = ip,
+    }
+  })
+end
+
+-- ##############################################
+
+local function triggerRttAlert(numeric_ip, ip_label, current_value, upper_threshold)
+  local entity_info = alerts_api.pingedHostEntity(ip_label)
+  local type_info = pingIssuesType(current_value, upper_threshold, numeric_ip)
+
+  return alerts_api.trigger(entity_info, type_info)
+end
+
+-- ##############################################
+
+local function releaseRttAlert(numeric_ip, ip_label, current_value, upper_threshold)
+  local entity_info = alerts_api.pingedHostEntity(ip_label)
+  local type_info = pingIssuesType(current_value, upper_threshold, numeric_ip)
+
+  return alerts_api.release(entity_info, type_info)
+end
+
+-- #################################################################
+
+-- Defines an hook which is executed every minute
+function script.hooks.min(params)
+  local hosts = rtt_utils.getHosts()
+  local pinged_hosts = {}
+  local max_latency = {}
+  local when = params.when
+
+  if(do_trace) then
+     print("[RTT] Script started\n")
+  end
+
+  if table.empty(hosts) then
+    return
+  end
+
+  for key, host in pairs(hosts) do
+     local host_label = host.host
+     local ip_address = host_label
+     local is_v6
+
+     if host.iptype == "ipv6" then
+	is_v6 = true
+     else
+	is_v6 = false
+     end
+     
+     if not isIPv4(host_label) and not is_v6 then
+       ip_address = ntop.resolveHost(host_label, true --[[IPv4 --]])
+
+       if not ip_address then
+          if do_trace then
+             print("[RTT] Could not resolve IPv4 host: ".. host_label .."\n")
+          end
+	  goto continue
+       end
+     elseif not isIPv6(host_label) and is_v6 then
+	ip_address = ntop.resolveHost(host_label, false --[[IPv6 --]])
+
+	if not ip_address then
+          if do_trace then
+             print("[RTT] Could not resolve IPv6 host: ".. host_label .."\n")
+          end
+	  goto continue
+	end
+     end
+
+     if do_trace then
+	print("[RTT] Pinging "..ip_address.."/"..host_label.."\n")
+     end
+
+     ntop.pingHost(ip_address, is_v6)
+     pinged_hosts[ip_address] = key
+     max_latency[ip_address]  = host.max_rtt
+
+     ::continue::
+  end
+
+  ntop.msleep(2000) -- wait results
+  
+  local res = ntop.collectPingResults()
+
+  if(res ~= nil) then
+     for host, rtt in pairs(res) do
+	local max_rtt = max_latency[host]
+	local key     = pinged_hosts[host]
+
+	if(do_trace) then
+	   print("[RTT] Reading response for host ".. host .."\n")
+	end
+
+	if params.ts_enabled then
+	   ts_utils.append("monitored_host:rtt", {ifid = getSystemInterfaceId(), host = key, millis_rtt = rtt}, when)
+	end
+
+	rtt = tonumber(rtt)
+	rtt_utils.setLastRttUpdate(key, when, rtt, host)
+	
+	if(max_rtt and (rtt > max_rtt)) then
+	  if(do_trace) then print("[TRIGGER] Host "..host.."/"..key.." [value: "..rtt.."][threshold: "..max_rtt.."]\n") end
+	  triggerRttAlert(host, key, rtt, max_rtt)
+	else
+	  if(do_trace) then print("[OK] Host "..host.."/"..key.." [value: "..rtt.."][threshold: "..max_rtt.."]\n") end
+	  releaseRttAlert(host, key, rtt, max_rtt)
+	end
+	
+	pinged_hosts[host] = nil -- Remove key
+     end
+  end
+
+  for ip,label in pairs(pinged_hosts) do
+     if(do_trace) then print("[TRIGGER] Host "..ip.."/"..label.." is unreacheable\n") end
+     triggerRttAlert(ip, label, 0, 0)
+  end
+
+  if(do_trace) then
+     print("[RTT] Script is over\n")
+  end
+end
+
+-- #################################################################
+
+return script