Compute Admin image guide refactor

Removed old content about creating custom images from the Compute
Admin Guide, added a pointer to the image guide.

Moved section about capturing images from running instances from
the Compute Admin Guide to the CLI guide.

Cherry-picked from https://review.openstack.org/31911

Change-Id: I4a7436b8d928e3fb18a07ec8234253c32cb3b6d9
Merge "Compute Admin image guide refactor" into stable/grizzly