CBQE-0: ability to set eviction_policy for xdcr tests

Change-Id: I1f85ac93f8253331479dec8bd017bd4460e966b1
Reviewed-on: http://review.couchbase.org/36600
Reviewed-by: Aruna Piravi <aruna@couchbase.com>
Tested-by: buildbot <build@couchbase.com>