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 runningi nstances from
the Compute Admin Guide to the CLI guide.

Rebase against master to try to merge.

Change-Id: I4a7436b8d928e3fb18a07ec8234253c32cb3b6d9