[1246251] MGMT, Storage nets have no gateway

Management and storage networks must have no gateway for both
Neutron and Nova-Network providers.
Floating network must have no gateway in case of Nova-Network.
Gateway field is set to 'None' for them.
'network_size' field is relevant for fixed network only.
Small refactoring of networking-related tests.

Closes-Bug: #1246251
Change-Id: Icfa425c094e3d0c45b0a9e979cdf3f0c719d106d
Merge "[1246251] MGMT, Storage nets have no gateway"