roles/collectd: Support setting server interface

For hosts with multiple network interfaces, collectd may not send
multicast messages through the correct interface.  To ensure that it
does, the `Interface` configuration option can be specified with each
`Server` option.  To define this option, entries in the
`collectd_network_servers` list can now have an `interface` property.

roles/collectd/templates/collectd-network.conf.j2

@@ -6,6 +6,10 @@ LoadPlugin network
 {% for server in collectd_network_servers %}
 {% if server is string %}
     Server "{{ server }}"
+{% elif server.interface is defined %}
+    <Server "{{ server.host }}"{% if server.port is defined %} {{ server.port }}{% endif %}>
+        Interface "{{ server.interface }}"
+    </Server>
 {% else %}
     Server "{{ server.host }}" {{ server.port }}
 {% endif %}