MB-11588: don't enable replica index by default with no replicas

We used to default to replica index enabled when incoming REST request
doesn't specify replica index flag. We now default to false if 0
replicas is configured.

Change-Id: Idacae143d09e378681715b90810d8771db0d94ca
Reviewed-on: http://review.couchbase.org/39555
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>