MB-11284: Support "admin" user in memcached core

Change-Id: I8e990e147ea870857483638bd1e9fa8e6fea551b
Reviewed-on: http://review.couchbase.org/37830
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>