Add ability to change primary user group.

Add ability to change primary user group in the environment.

        "developer": {
          "enabled": false,
          "manage_home": true,
          "sudo": true,
          "password": "devPASSword1111",
          "note": "developer",
          "home": "/home/developer",
          "shell": "/bin/bash",
          "group": "primary",
          "groups": [
            "www-data",
            "root",
            "developer",
            "somethingnew"
          ],
          "authorized_keys": [
            "keyhere"
          ]

Tested on Debian 7.2 and Ubuntu 12.04.