MB-12067: Enable defragmenter task by default

Change-Id: I7c342aeb306332b7ab389db18f457453c6506e1a
Reviewed-on: http://review.couchbase.org/44423
Reviewed-by: Jim Walker <jim@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Dave Rigby <daver@couchbase.com>