CBQE-0: add param extra_buckets in XDCRBaseTestCase

this param is required for the cases when inside test we add more buckets those that have been added in setup
(avoid "ramQuotaMB":"RAM quota specified is too large to be provisioned into this cluster)

Change-Id: I67ac1e6f43ff958e752f132583964af27e1c8751
Reviewed-on: http://review.couchbase.org/36831
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>