MB-8665: optimize is_system_provisioned

This is called from /pools and we have to assume sdk's touch this
often.

In my measurements this cut about 1/3rd of time it takes to render
/pools.

Change-Id: I219ce62064e5864bbb375d4cd3050edbf7417e50
Reviewed-on: http://review.couchbase.org/28951
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>