Add a little more test coverage for rbd resize logic
Add a little more test coverage for rbd resize logic

Since the resize decisions are packed tight into a helper, it
is trivial to test the boundary conditions alone. This adds a test
to do that, including the previously-uncovered condition of the
8G resize ceiling.

Change-Id: I04b4e2f220cf93275f0b14240cbbfff3da94e4d1

openstack/glance_store,openstack/glance_store