Fix _check_network_external_connectivity in test_network_basic_ops

TestNetworkBasicOps.test_network_basic_ops failed when there were
more than one ipv4 subnets in public network. This patch is to fix
it.

Change-Id: I1d385528afb600b8e6131cb59905ba1e8eff130d
Closes-Bug: #1637050
1 file changed