NCBC-589: ThrowInvalidOperationException in CouchbaseConfigContext.LoadConfig during rebalance

Change-Id: I813092f07bb6fde414ea4e05a67239173a540568
Reviewed-on: http://review.couchbase.org/40715
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>