Consolidate _check_public_network_connectivity into NetworkScenarioTest

test_network_advanced_server_ops.py and test_network_basic_ops.py both
have similar _check_public_network_connectivity methods that do the same
thing but handle errors slightly different.

This change consolidates the common code in the parent class and uses
the more robust error handling / diagnostic debug logging of the two.

A follow-on change will do the same for
_check_tenant_network_connectivity.

Partial-Bug: #1323674

Change-Id: I412c9949fb99df950fdde4daab4b1b4f34396d56
Merge "Consolidate _check_public_network_connectivity into NetworkScenarioTest"