MB-8872 check basic auth for _view_merge requests

for backwards compat with pre 3.0.2 clusters allow anonimous access
if not all of the nodes have 3.0.2 or higher version

Change-Id: Ib14e5fd4a9d18de92893dd89a936f8f03f2727f8
Reviewed-on: http://review.couchbase.org/43376
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>