Group mgradm commands

[cbosdo]

Leverage the feature implemented in spf13/cobra#1003 to group the subcommands in mgradm as those start to be numerous.

[mbussolotto]

I try to group the current subcommand. Let me know what you think about it:

• Deploy:
  • install
  • migrate
  • uninstall
  • hub
• System Management:
  • restart
  • start
  • status
  • stop
• Tools:
  • distribution
  • gpg
  • inspect

[cbosdo]

How about this?

 * **Server deployment:**
   
   * install
   * migrate
   * uninstall
   * inspect

 * **Server management:**
   
   * restart
   * start
   * status
   * stop

 * **Administrator tools:**
   
   * distribution
   * gpg
   * hub