Allow user to override UseDNS in template

You could perhaps have something that overrides a container network
like this;

---
container_extra_networks:
  dns_address:
    ...
    dhcp_use_dns: false

and would take precedence over lxc_container_enable_resolved

Also documentations says UseDNS takes true/false

https://www.freedesktop.org/software/systemd/man/systemd.network.html#UseDNS=

Change-Id: I91f25eaf0782a6b4febfc95d9ee7c33f1f965224

templates/container_network.network.j2

@@ -7,7 +7,11 @@ Name={{ item.value.interface }}
 Address={{ item.value.address }}/{{ addr_cidr }}
 {% else %}
 [DHCP]
+{% if item.value.dhcp_use_dns is defined %}
+UseDNS={{ item.value.dhcp_use_dns | bool | lower }}
+{% else %}
 UseDNS={{ (lxc_container_enable_resolved | bool) | ternary('yes', 'no') }}
+{% endif %}
 UseNTP=yes
 RouteMetric=20
 {% if hostvars[inventory_hostname]['gateway'] is defined %}