ostree: provide command description in a better place

ostree: provide command description in a better place

This is similar idea as
https://github.com/projectatomic/rpm-ostree/pull/916/commits/5c0bf889155573feec108501ce849ac09a0c9567,

The duplicated description is now removed, and the description
of the command is now displayed beneath the Usage.

For example:
ostree cat -h will output the following:

"Usage:
  ostree cat [OPTION?] COMMIT PATH...

Concatenate contents of files"

Closes: #1267
Approved by: cgwalters