[OSTF] Health check failed if other external network without router exists

[Michael Kraynov]

If we add one or more external network to the cluster without router the health check "Check network connectivity from instance via floating IP" will fail with error "Can not assign floating ip to instance Please refer to OpenStack logs for more details"

Steps to reproduce:
1. Deploy environment
2. Add external network
neutron net-create ext2 -- --router:external=true --provider:network_type=flat --provider:physical_network=phyext2
neutron subnet-create --allocation-pool start=172.20.33.100,end=172.20.33.199 ext2 172.20.33.0/24
3. Run health check

Expected results:
Health check uses default external network and pass the test

Actual result:
Health check failed

Description of the environment:
MOS 9.0