MB-100 Implement just_validate for global settings too.

Change-Id: I4466f313afe745d101ad378598bd20039fce993d
Reviewed-on: http://review.couchbase.org/28369
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>