NCBC-253: Raise exception on FlushAll when called on CouchbaseNode

Change-Id: I910a9f903af65ccbb0a3656873ca1d6f4bb4c12d
Reviewed-on: http://review.couchbase.org/26268
Reviewed-by: Saakshi Manocha <saakshi.manocha@globallogic.com>
Tested-by: Saakshi Manocha <saakshi.manocha@globallogic.com>