[enhancement] allows using multiple web listen addresses

fixes #115

Signed-off-by: Christian Krause <christian.krause@mailbox.org>

roles/alertmanager/templates/alertmanager.service.j2

@@ -33,7 +33,15 @@ ExecStart={{ _alertmanager_binary_install_dir }}/alertmanager \
 {% endfor %}
   {{ pre }}-config.file={{ alertmanager_config_dir }}/alertmanager.yml \
   {{ pre }}-storage.path={{ alertmanager_db_dir }} \
+{% if alertmanager_web_listen_address is iterable and
+      alertmanager_web_listen_address is not mapping and
+      alertmanager_web_listen_address is not string %}
+{%   for address in alertmanager_web_listen_address %}
+  {{ pre }}-web.listen-address={{ address }} \
+{%   endfor %}
+{% else %}
   {{ pre }}-web.listen-address={{ alertmanager_web_listen_address }} \
+{% endif %}
   {{ pre }}-web.external-url={{ alertmanager_web_external_url }}{% for flag, flag_value in alertmanager_config_flags_extra.items() %} \
   {{ pre }}-{{ flag }}={{ flag_value }}{% endfor %}