vbucket: add lcbvb_genconfig_ex()

This allows to generate a configuration handle using more detailed input
parameters. Provided to better test forwarding emulation

Change-Id: Ic9a767b72aa4536737ffe7eaf2c0f506b7c0c280
Reviewed-on: http://review.couchbase.org/37784
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>