Added local_admin_users resouce

A straight forward coding using [chef's group resource](http://docs.opscode.com/resource_group.html)
Tested successfully with the following chef-json input:

```json
{
    "run_list": "recipe[gecos_ws_mgmt::default]",
    "gecos_ws_mgmt": {
        "misc_mgmt": {
            "local_admin_users_res": {
                "local_admin_list": ["ami", "vagrant", "jonas"]
            }
        }
    }
}
```