CBQE-0: replace self._timeout on self.wait_timeout in xdcr related tests

Now there is a lot of confusion in the names of variables
when we extend tests classes from xdcr and basetestcases

Change-Id: Ifc41f2c21c1dc7e948446aa0930a217f8770fb07
Reviewed-on: http://review.couchbase.org/34882
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>