MB-36926: Do not always attempt to run RocksDB flush benchmark
MB-36926: Do not always attempt to run RocksDB flush benchmark

We may not always be building with RocksDB so do not attempt to
run the flusher memory tracking benchmark when we do not build with
RocksDB.

Change-Id: I6dde50a4e50268089b9d425f2cfaa8bcedbdd0df
Reviewed-on: http://review.couchbase.org/118970
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine