create groups for mgradm subcommands

[mbussolotto]

What does this PR change?

create groups for mgradm subcommands. This is how it looks:

sumacontainer:~ # ./mgradm --help
Tool to help administering Uyuni servers in containers

Usage:
  mgradm [command]

Server Deployment:
  inspect      Inspect
  install      Install a new server
  migrate      Migrate a remote server to containers
  uninstall    Uninstall a server
  upgrade      Upgrade local server

Server Management:
  hub          Hub management
  restart      Restart the server
  start        Start the server
  status       Get the server status
  stop         Stop the server

Administrator tools:
  distribution Distributions management
  gpg          Manage GPG keys for 3rd party repositories
  support      Commands for support operations

Additional Commands:
  help         Help about any command

Flags:
  -c, --config string     configuration file path
  -h, --help              help for mgradm
      --logLevel string   application log level(trace|debug|info|warn|error|fatal|panic)
  -v, --version           version for mgradm

Additional help topics:
  mgradm config       Help on configuration file and environment variables

Use "mgradm [command] --help" for more information about a command.

Test coverage

• Unit tests were added

• DONE

Links

Issue(s): #274

• DONE

Changelogs

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