Add 'bash-completion' to 'ironic help' response

If you do 'ironic' or 'ironic help', it will show a list of subcommands.
This adds 'bash-completion' to that list.

The 'ironic bash-completion' subcommand lists all of the commands.
It won't include 'bash-completion' in this list, to be consistent with
nova's and keystone's bash-completion subcommand.

Change-Id: I7f2b76ad2885d9fe34dc91a97ee77984a21a27f4
Closes-Bug: #1304488
Merge "Add 'bash-completion' to 'ironic help' response"