added scripts/vbhash.rb that converts key into vbucket number

Because we lack such tool and having such tool is super-useful for
xdcr debugging

Change-Id: Ib456a98ef816e5ed4a192ed547514663c87a632d
Reviewed-on: http://review.couchbase.org/35122
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>