proxmox: add initial support for L2 VXLAN

I heard we like L2 so I put some L2 in our L3 so we can L2 as we L3 on L2.

roles/proxmox/templates/interfaces.j2

@@ -0,0 +1,30 @@
+# Keep vmbr0 named as is and in the main interfaces file so Proxmox can find it.
+
+# Bridge for V(X)LANs.
+auto vmbr0
+iface vmbr0 inet manual
+    bridge-vlan-aware yes
+    bridge-ports {{ vlans | map('regex_replace', '^', 'vni') | join(' ') }}
+    bridge-stp off
+    bridge-fd 0
+
+# Interfaces.
+{% for vlan in vlans %}
+auto vni{{ vlan }}
+iface vni{{ vlan }} inet static
+    vxlan-id {{ vlan }}
+    bridge-access {{ vlan }}
+    mstpctl-bpduguard yes
+    mstpctl-portbpdufilter yes
+
+{% endfor %}
+
+# In place of vni* interfaces above this should work also but does not.
+# Might start working after proxmox upgrades their ifupdown2.
+#auto vxlan
+#iface vxlan inet static
+#    bridge-vlan-vni-map {{ vlans | zip(vlans) | map('join', '=') | join(' ') }}
+#    bridge-vids {{ vlans | join(' ') }}
+#    bridge-learning off
+
+source /etc/network/interfaces.d/*