CBQE-1913: moving failover tests to subdir

creates failover test module and separates base test class
for inheritance by other tests

Change-Id: I68921caa730dc44294c67ecc55a03e38ad03309e
Reviewed-on: http://review.couchbase.org/32771
Reviewed-by: Aruna Piravi <aruna@couchbase.com>
Tested-by: buildbot <build@couchbase.com>