Add sync_ceph_time method to all Ceph tests

Sometimes after reverting snapshots Ceph health check
alerts about clock skew on some nodes, so an additional
time sync and ceph service restart were added to the
test. Also, refactored methods which are used for ceph
checks.

Change-Id: I6c875f19cd3802e8280de48d121b7900b25a0f3a
Closes-bug: #1349379
(cherry picked from commit 3c7a8aa6e35d9e4ae36c8b2ae3ea39baec76a211)
Merge "Add sync_ceph_time method to all Ceph tests" into stable/5.0