TrivialFix: Delete virtual bridge artifacts when cleaning hosts

After cleaning hosts by using tools/cleanup_host, some of the ovs bridge
devices such as ovs-system still exist. This will result in
{{neutron_external_interface}} not working properly because it had been
added to the ovs-system bridge. The solution is calling
neutron-ovs-cleanup script in neutron_openvswitch_agent container before
removing it.

TrivialFix

Change-Id: Ib3b096d842f2210b8bd223892a3492ef2fcf7c52
Signed-off-by: luyao <lu.yao135@zte.com.cn>