Fixed bypass of Public GW check

When Public network has only one IP range set
the check of intersection between GW and IP ranges of Public network was skipped.
Both Nova-Net and Neutron. It's fixed here.
Tests are added. Slight tests refactoring is made.

Closes-Bug: #1273175

Change-Id: I0f6ca9e43a5c21aebefb9b38f9b8cb59c02126fe
Merge "Fixed bypass of Public GW check"