[PRD-2520] Public network is untagged by default

Nova-Network default network assignment was changed.
Now it is the same as in case of Neutron provider.

Some little refactoring is made in NetworkManager/NeutronManager.

Change-Id: I41b1c0c32421669dd75629a3d63d171a88d50bc4
Merge "[PRD-2520] Public network is untagged by default"