CBQE-1407: test_failover_firewall doesn't work properly on windows

add GROUP=WINDOWS in conf/py-failover.conf

Change-Id: I4c85ef66bfbaee23b4f396530ecbe19e0e86761f
Reviewed-on: http://review.couchbase.org/27100
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>