Add help for purge_deleted command age argument

At a bare minimum explaining this argument helps understand what
purge_deleted is going to do.

Change-Id: If73ba272e8ea0742f8be93994ec2fac68d4ed3fd
Add cinder prop image, deprecate imageRef

This change does a lookup on the provided image so that
an image name or UUID can be specified in the property.

imageRef has been documented as being deprecated.

Closes-Bug: #1234263
Change-Id: I95190e3f53ddac8eb41306b7003aa93e75886eb7
Merge "Add cinder prop image, deprecate imageRef"
Merge "Add help for purge_deleted command age argument"