MB-8882 Don't ask read-only user to create cluster references.

Because it's impossible anyway.

Change-Id: Ia591d57afcd0e0c56a0851d2ccf366b4a71b557a
Reviewed-on: http://review.couchbase.org/28414
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>