Display gdk help when gdk is called without args

What does this Merge Request do?

This MR updates the gdk command to display the help content when gdk is called without args:

Prior to this MR

$ gdk
(in /Users/ash/src/gitlab/gitlab-development-kit)
=> gdk:  is not a gdk command.
=> See 'gdk help' for more detail.

With this MR

$ gdk
(in /Users/ash/src/gitlab/gitlab-development-kit)


           `                        `
          :s:                      :s:
         `oso`                    `oso.
         +sss+                    +sss+
        :sssss:                  -sssss:
       `ossssso`                `ossssso`
       +sssssss+                +sssssss+
      -ooooooooo-++++++++++++++-ooooooooo-
     `:/+++++++++osssssssssssso+++++++++/:`
     -///+++++++++ssssssssssss+++++++++///-
    .//////+++++++osssssssssso+++++++//////.
    :///////+++++++osssssssso+++++++///////:
     .:///////++++++ssssssss++++++///////:.`
       `-://///+++++osssssso+++++/////:-`
          `-:////++++osssso++++////:-`
             .-:///++osssso++///:-.
               `.://++osso++//:.`
                  `-:/+oo+/:-`
                     `-++-`


# GitLab Development Kit

Usage: gdk <command> [<args>]

Manage services:

  gdk start                                 # Start everything
  gdk start redis postgresql                # Start specific services
  gdk stop                                  # Stop all services and unload Runit
--snip--

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.