ostree: Describe subcommands in help output

ostree: Describe subcommands in help output

Added a description argument to all type
of commands. Now when we include -h or --help
for commands that contain subcommands, the description
for those subcommands are shown.

The added subcommands help will be provided to the following commands:
- ostree -h
- ostree admin -h
- ostree admin instutil -h
- ostree remote -h
- ostree static-delta -h

Closes: #1267
Approved by: cgwalters