Correctly support list of string prefixes

systemd_networks:
  - interface: dummy3
    address:
      - 10.0.2.100/24
      - 10.0.3.100/24

Change-Id: If2f08ff70b637090f0ed1091bd40ce090468a51a

templates/systemd-network.j2

@@ -31,6 +31,9 @@ Metric={{ 20 + loop.index }}
 
 [Network]
 {% for _addr in _addresses %}
+{%   if _addr is string %}
+{%       set _addr = {'address': _addr} %}
+{%   endif %}
 {%   if _addr.address == 'dhcp' %}
 DHCP=yes
 {%   else %}