reply /pools/default/certificate with raw certificate PEM

Rather than wrapping it in json which is less convenient

Change-Id: I98a418c2b4eb388f6841cf2807e99c4b64ead39a
Reviewed-on: http://review.couchbase.org/31802
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>