Ensure we don't access the net when building docs

(Note, this has not been sent upstream)

Change-Id: I9d02fb4053a8106672aded1614a2850e21603eb2
Check QCOW2 image size during root disk creation

glance can only tell us the size of the file, not the virtual
size of the QCOW2. As such we need to check the virtual size of
the image once its cached and ensure it's <= to the flavor's
root disk size.

Change-Id: I833467284126557eb598b8350a84e10c06292fa9
Fixes: bug 1177830
(cherry picked from commit 44a8aba1d5da87d54db48079103fdef946666d80)