Increase CEPH_LOOPBACK_DISK_SIZE for ceph

check-tempest-dsvm-full-ceph is failing due to space
contraints in the Ceph store.  Increase the size of this
store to alleviate this problem.

Change-Id: I753cee181b7a4e4de36840407f9cfbaafc4802a6
Merge "Increase CEPH_LOOPBACK_DISK_SIZE for ceph"