Add support for VLANs

[fw2568]

This PR adds support for VLAN tagging of overlay provider networks.

Provider VLANs

New option vlan in network provider configuration:

network_providers:
- name: default
  type: overlay
  vlan: 10

Overlay provider network VLAN tags are attached by OVN to all egress traffic from the VMs. This is accomplished by configuring the tag to the logical switch port of the external router port that connects from the project network to the provider network.

Bridge VLANs and IP mode

The bridge adapter is not affected by the provider's VLAN setting. The default configuration for an overlay adapter also doesn't enable the bridge adapter for network communications.

For scenarios where we have few (or only one) network adapters, the bridge can now be configured to be enabled for DHCP by default. For this case, we now also have the option to configure a VLAN for the bridge adapter:

network_providers:
- name: default
  type: overlay
  vlan: 10
  bridge_name: br-pif
  bridge_options: 
   default_ip_mode: dhcp
   bridge_vlan: 11

See also #144 for static address configuration support.