engine : Updating host's cluster leads to cloning the host

When trying to move host from one gluster cluster to
another, under certain conditions the operation fails
with an internal server error and the host appears in
both clusters.

This happens only when the host in the target or
source cluster was up during canDoAction and is down
during execution of glusterHostRemove or
glusterHostAdd restulting in NPE.

Change-Id: I1bda0efedd62bf70ae8670861dc3a2ee70ff77dc
Bug-Url: https://bugzilla.redhat.com/988033
Signed-off-by: Ravi Nori <rnori@redhat.com>