Make size of loop device used by cinder LVM configurable

This was hard-coded to 5 gig, which is useless for anything other
than tempest runs and smoke testing

block-storage-nfs.yaml has intentionally not been changed, since
volume_size_mb is not used in that setup. Cleaning up that code will
be done separately.

Change-Id: I476b906a8d439d3e6643dd0c214965c5862418e8
Merge "Make size of loop device used by cinder LVM configurable"