[Fullstack] Ensure connectivity to ext gw before agents restart
[Fullstack] Ensure connectivity to ext gw before agents restart

In TestHAL3Agent.test_ha_router_restart_agents_no_packet_lost
fullstack test we should first ensure that connection from external_vm
to router's external gateway is possible. If it's fine, we
can restart L3 agents and test if connectivity will not be broken.

Change-Id: I1f153c553cd2dfa846ce80c166e2a35acd9169a3
Related-Bug: #1776459

huntxu/neutron,openstack/neutron,mahak/neutron,mahak/neutron,mahak/neutron,huntxu/neutron,openstack/neutron,openstack/neutron,noironetworks/neutron,noironetworks/neutron