MB-100: bump priority of ns_doctor

Change-Id: Id3e34079cefc6a02b014e2a58df66472da3061f0
Reviewed-on: http://review.couchbase.org/27587
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>