New endpoint for restarting agents by group

[druizz90]

Hi team,

This PR closes #411. I added a new endpoint for restarting agents by group:

# curl -u foo:bar -k -X PUT "http://127.0.0.1:55000/agents/groups/default/restart?pretty"
{
   "error": 0,
   "data": {
      "msg": "All selected agents were restarted",
      "affected_agents": [
         "001",
         "002",
         "003"
      ]
   }
}

# curl -u foo:bar -k -X PUT "http://127.0.0.1:55000/agents/groups/dmz/restart?pretty"
{
   "error": 1732,
   "message": "No agents selected"
}

# curl -u foo:bar -k -X PUT "http://127.0.0.1:55000/agents/groups/dmzzz/restart?pretty"
{
   "error": 1710,
   "message": "The group does not exist"
}

There are more details about mocha and unit tests in #411.

Best regards,

Demetrio.