MB-8729: Add get_engine_vb_map interface for CCCP

Allow the engine to specify the new vbucket map

Change-Id: I2459372d5c958445063d50a4c77c9058b0e34de9
Reviewed-on: http://review.couchbase.org/29845
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>