added option to run metakv sanity test at runtime

So that we can easily verify that metakv works against running
ns_server. I.e. is is as simple as setting environment variable
COUCHBASE_METAKV_SANITY=1 and starting couchbase server.

Change-Id: I0f91ffae13020896d8f8ee1f5b8c97ec236a193b
Reviewed-on: http://review.couchbase.org/44552
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>