Support for Nova-Network

There are still real use-cases for nova-network, for instance Nova with VMWare/VCenter driver. Because Heat's Neutron resources are not backward-compatible with pre-Neutron (nova-network) setups deployment of Murano environments that do use those resources always fail.
The solution is to make core library aware of it Neutron is present or not and use different resource types to setup networking. Murano API can query keystone for Neutron presence and configure networking in Object Model appropriately.