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/ghost_networks/user_scripts/interface/ghost_networks.lua

@@ -0,0 +1,39 @@
+--
+-- (C) 2019 - ntop.org
+--
+
+local alerts_api = require("alerts_api")
+local alert_consts = require("alert_consts")
+local script
+
+-- #################################################################
+
+local function check_ghost_networks(params)
+  for domain, domain_info in pairs(params.entity_info.bcast_domains or {}) do
+    if(domain_info.ghost_network) then
+      local key = params.user_script.key .. "__" .. domain
+      local delta_hits = alerts_api.interface_delta_val(key, params.granularity, domain_info.hits)
+      local ghost_network_type = alerts_api.ghostNetworkType(domain, params.granularity)
+
+      if(delta_hits > 0) then
+        alerts_api.trigger(params.alert_entity, ghost_network_type, nil, params.cur_alerts)
+      else
+        alerts_api.release(params.alert_entity, ghost_network_type, nil, params.cur_alerts)
+      end
+    end
+  end
+end
+
+-- #################################################################
+
+script = {
+  default_enabled = true,
+
+  hooks = {
+    min = check_ghost_networks,
+  },
+}
+
+-- #################################################################
+
+return script