Provide support for Provider Networks (VLAN & Flat) and DHCP disable capability

Currently the heat translator provides for creating a VXLAN network_type as defined by the tosca.nodes.network.Network but does not support the creation of provider networks (VLAN or flat). Adding support for provider networks would allow for addition types of network connectivity to VDUs. In addition OS:Neutron::Subnet allows for disabling of a DHCP server through the 'enable_dhcp'; property which maps nicely to the TOSCA NFV 'dhcp_enable' property.

The new NETWORK_PROPS being proposed are: 'network_type' and 'physical_network'
The new SUBNET_PROPS being proposed is: 'dhcp_enable'