MB-100: implemented assert_is_enterprise/0

Change-Id: Ieec5d79179d4032bf39170bac7dfed9dd70431ed
Reviewed-on: http://review.couchbase.org/30152
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>