Skip cluster events when no cluster exists

When a host is updated in EMS Refresh, it's possible for a residual EMS Cluster
ID to exist on the host.  The only scenario that can show this is when an EMS is
deleted and then re-added.

When an EMS is deleted, the hosts for the EMS are *not* deleted.  And, in order
to avoid a performance hit, the ems_cluster_id (belonging to a deleted EMS
Cluster) is not removed from the host record.

When the EMS is re-added, the EMS Refresh code attempts to update the host
record with the new EMS Cluster ID (as well as all the other attributes from the
refresh).

When the EMS Cluster id is updated on the host, an after_save callback is
initiated that attempts to emit events for the old EMS Cluster being removed and
the new EMS Cluster being added.

However, when the after_save callback is attempted for the (now non-existent)
old EMS Cluster, active record fails to find the record and throws a
RecordNotFound error.

This fix allows the host to update its attributes and ignore a residual EMS
Cluster ID in the after_save callback if necessary.

https://bugzilla.redhat.com/show_bug.cgi?id=1087476