New call for deleting multiple agents of a group

[druizz90]

Hi team,

This PR is for issue #258. We added a new call to insert a list of agents to a group in PR #256. This call is for removing a list of agents from a group:

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

curl -u foo:bar -X DELETE -H "Content-Type:application/json" -d '{"ids":["001","002"]}' "http://localhost:55000/agents/group/dmzz?pretty"
{
   "error": 1710,
   "message": "The group does not exist"
}

Best regards,

Demetrio.