Refactor the MockCommand interface

Pass the mock to the execute method to avoid implementing
a constructor, and return the response as part of the
execute method.

Change-Id: Icf437d08e076f541fad700d27e7b2fff3344c2e1
Reviewed-on: http://review.couchbase.org/27954
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Trond Norbye <trond.norbye@gmail.com>