Increase CEPH_LOOPBACK_DISK_SIZE_DEFAULT to 4GB

CEPH_LOOPBACK_DISK_SIZE_DEFAULT should be more than 2GB
to make volume snapshot feature works. 2GB is not enough
because min Cinder volume size is 1GB and no snapshot
could be created.

This also fixes related Tempest tests and experimental
check-tempest-dsvm-full-ceph gate job.

Change-Id: Ifa41d0d1764d68ea02dcb32a5fc62f7f6282904d
Merge "Increase CEPH_LOOPBACK_DISK_SIZE_DEFAULT to 4GB"