auto refresh tree and metrics&events panels when cluster added/removed

Change-Id: I418bb739f5e9b293d6aad5f2382e5e52bccafdb3
Reviewed-on: http://review.couchbase.org/23825
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>