New call for adding multiple agents to a group

[druizz90]

Hi team,

This PR is for issue #254. I added a new call for adding multiple agents to a group in the same call:

$ curl -u foo:bar -X POST -H "Content-Type:application/json" -d '{"ids":["001","002"]}' "http://localhost:55000/agents/group/dmz?pretty"
{
   "error": 0,
   "data": {
      "msg": "All selected agents assigned to group dmz",
      "affected_agents": [
         "001",
         "002"
      ]
   }
}

$ curl -u foo:bar -X POST -H "Content-Type:application/json" -d '{"ids":["003","007"]}' "http://localhost:55000/agents/group/dmz?pretty"
{
   "error": 0,
   "data": {
      "msg": "Some agents were not assigned to group dmz",
      "failed_ids": [
         "007"
      ],
      "affected_agents": [
         "003"
      ]
   }
}

$ curl -u foo:bar -X POST -H "Content-Type:application/json" -d '{"ids":[]}' "http://localhost:55000/agents/group/dmz?pretty" 
{
   "error": 1732,
   "message": "No agents selected"
}

Best regards,

Demetrio.