Added support for returning owner's group name in stat module #8967.

Added support for returning owner's group name in stat module.

This would enable Ansible users to use stat module to get the group name of a file or a directory. The current stat module is only able to get the gid.

Usage:
    - name: "Get a file's group name"
      stat: path="/tmp/somefile"
      register: st
    
    - debug: var=st.gr_name