Generalize is_projector_available function.

Change-Id: Ia8e62ee649a3403958b636b260236a13d3e2e099
Reviewed-on: http://review.couchbase.org/43813
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>