[System Tests] Need to add test to check connectivity after neutron agents migrations

Test steps will be the next:
1. Create neutron gre cluster, 3 controllers, 2 compute, 1 cinder and deploy
 2. Create instance and assign floating ip to it
 3. Add collocation between l3-agent and dhcp-agent
 4. Create agents_migration snapshot
 5. 10 times revert snapshot agents_migration
 6. Migrate manually dhcp-agent and l3-agent
 7. Check ping to floating ip
 8. In case of failure increase number of failures
 9. Restart openvswitch-agent on node with l3 agent
 10. Ping floating ip again
 11. In case of success increase number of workarounds on 1
 12. In case of failure increase number of failed workarounds on 1

This test is only actual for 5.1.1 branch. On 6.0 we have multiple l3 agents and case will be different