alpine: support non-VM hosts in interfaces template

Ignore OOB management interface, allow configuring loopback interface
with NetBox data, and setting MTU.

roles/alpine/templates/interfaces.j2

@@ -1,9 +1,21 @@
+{# Loopback interface must be present so define it here if none exists. #}
+{% if interfaces | rejectattr("name", "==", "lo") %}
 auto lo
 iface lo inet loopback
 
-{% for iface in interfaces | selectattr('enabled') %}
+{% endif -%}
+
+{# Skip disabled and OOB management interfaces. #}
+{# For VMs we have to set the attribute manually (to false) so rejectattr works. #}
+{% for iface in interfaces
+    | defaultattr('mgmt_only')
+    | rejectattr('mgmt_only')
+    | selectattr('enabled') %}
 auto {{ iface.name }}
-iface {{ iface.name }} inet static
+iface {{ iface.name }} inet {% if iface.name == "lo" %}loopback{% else %}static{% endif +%}
+{% if iface.mtu %}
+    mtu {{ iface.mtu }}
+{% endif %}
 {% for address in iface.ip_addresses %}
     address {{ address.address }}
 {% if address.family.value == 4 %}
@@ -21,4 +33,6 @@ iface {{ iface.name }} inet static
     pre-up echo 0 > /proc/sys/net/ipv6/conf/$IFACE/autoconf
 {% endif %}
 
-{% endfor %}
+{% endfor -%}
+
+source-directory /etc/network/interfaces.d