MB-11932: made is_enterprise config key per-node

Which enables meaningful mix of CE and EE nodes and enables rebalance
up- and down- grade.

Change-Id: If0b3a41a8a5c5c6f7e8e202f0930b0c1f8b4ca44
Reviewed-on: http://review.couchbase.org/40515
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>