MB-7396 Don't open view groups without views

If a design document doesn't define any mapreduce views,
don't start a corresponding view group process, as it's
not useful and it only consumes resources (memory and file
descriptors).

Change-Id: Iba7b1fb512b40f2e9e4c3b2f2753deb5a2b7d045
Reviewed-on: http://review.couchbase.org/24032
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>