Introduce NeutronNetworkDeploymentSerializer

Neutron attributes serialization is moved from NetworkManager to
NeutronNetworkDeploymentSerializer.
Versioning is introduced in NeutronNetworkDeploymentSerializer.

The previous code in network/neutron.py checked specifically for 5.1
environments. This caused 6.0 environments to fall back to the
old behavior and set network_type to 'flat' and configure 'physnet'.

Change-Id: I3940d0910c888794f82dd2159727ef5073367916
Closes-Bug: #1357298
Merge "Introduce NeutronNetworkDeploymentSerializer"