engine: Gluster syncing lock correction- change cluster

While moving clusters - there is a "gluster peer detach"
as well as a "gluster peer probe", and we need to make sure
that it does not interfere with the gluster sync job that
updates servers in the cluster.

Introducing a lock before these 2 operations alone for
gluster lock.

Change-Id: Ibe28b5eb5baee573f0fa875e06d7ae945ffdb9f9
Signed-off-by: Sahina Bose <sabose@redhat.com>