MB-8665: introduced ns_config:config_version_token/0

This exposes quick way to detect if config has changed which we need
for menelaus_web_cache invalidation of pool details.

Change-Id: I376d841422c8a87c1e0ec8926831fa3f40e2104a
Reviewed-on: http://review.couchbase.org/28957
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>