Allow the user to add extra server config

apt-cacher/ng/files/server.conf

 CacheDir: {{ apt_cacher_ng.server_cache_dir }}
 LogDir: {{ apt_cacher_ng.server_log_dir }}
 Port: {{ apt_cacher_ng.server_port }}
+{% if apt_cacher_ng.server_extra_config -%}
+{{ apt_cacher_ng.server_extra_config }}
+{% endif -%}

apt-cacher/ng/map.jinja

         'server_address': 'localhost',
         'server_bind_address': '0.0.0.0',
         'server_port': '3142',
+        'server_extra_config': '',
         'server_config': '/etc/apt-cacher-ng/zzz_acng.conf',
         'server_cache_dir': '/var/cache/apt-cacher-ng',
         'server_log_dir': '/var/log/apt-cacher-ng',

pillar.example

   server_port: 3142
   server_cache_dir: /var/cache/apt-cacher-ng
   server_log_dir: /var/log/apt-cacher-ng
+  server_extra_config: |
+    # Workaround apt-cacher-ng 500 Invalid header
+    # https://lists.alioth.debian.org/pipermail/apt-cacher-ng-users/2013-September/000082.html
+    ReuseConnections: 0
 
   # Credentials for apt-cacher-ng Web management interface.
   # http://<server_adress>:<server_port>/acng-report.html