MB-8083: minor fix in launch_memcachetest

Change-Id: Ic799fd5ea84b63479bc2fde114e729cadca402f5
Reviewed-on: http://review.couchbase.org/37852
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: buildbot <build@couchbase.com>