caddy-frontend: Support not resolving backend hostnames

The setup "last,libc,none" allows lazy resolving of backend servers which is
crucial in order to being able to start when the domain does not exists (yet).

software/caddy-frontend/buildout.hash.cfg

@@ -50,7 +50,7 @@ md5sum = 37475d79f28c5f126bc1947fdb938fdb
 
 [template-backend-haproxy-configuration]
 _update_hash_filename_ = templates/backend-haproxy.cfg.in
-md5sum = 5e126be0f74d8ae390a5594e1e912a59
+md5sum = d2851c7ebd2c9baa2edecb3ca3485511
 
 [template-empty]
 _update_hash_filename_ = templates/empty.in

software/caddy-frontend/templates/backend-haproxy.cfg.in

@@ -13,6 +13,8 @@ defaults
   timeout client {{ configuration['request-timeout'] }}s
   timeout connect {{ configuration['backend-connect-timeout'] }}s
   retries {{ configuration['backend-connect-retries'] }}
+  {#- Allow to start with not resolved yet servers #}
+  default-server init-addr last,libc,none
 
 {%- set SCHEME_PREFIX_MAPPING = { 'http': 'http_backend', 'https': 'https_backend'} %}
 {%- macro frontend_entry(slave_instance, scheme, wildcard) %}

software/caddy-frontend/test/test.py

@@ -1310,6 +1310,9 @@ class TestSlave(SlaveHttpFrontendTestCase, TestDataMixin):
     return {
       'empty': {
       },
+      'bad-backend': {
+        'url': 'http://bad.backend/',
+      },
       'Url': {
         # make URL "incorrect", with whitespace, nevertheless it shall be
         # correctly handled