Consolidate _check_tenant_network_connectivity in NetworkScenarioTest

Moves _check_tenant_network_connectivity into the parent class for the
basic/advanced network scenario tests and keeps the better diagnostic
debug logging of the two in the common version.

Closes-Bug: #1323674

Change-Id: I4d3f7368e25405fc8baaf23ed1c95a65aef724b7
Merge "Consolidate _check_tenant_network_connectivity in NetworkScenarioTest"