Speedup memcached_testapp by splitting into plain and SSL

Add a new argument to memcached_testapp : plain/SSL. This allows each
half to be tested in parallel, speeding up the wallclock runtime (if
CTEST_PARALLEL_LEVEL is used).

Change-Id: Ic9bde8f36ce60c53ccfd507e8c93ba40f87abf42
Reviewed-on: http://review.couchbase.org/42715
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>