Cannot set maxconn for sites that don't have a backend

Hi,

If a site is serving local files (such as esm.ubuntu.com), it appears that with the current charm code, it's not possible to configure its maxconn, which will stay at a hardcoded value of 200 [1].

This is a problem, especially because a site service local files should be able to manage quite more than that.

Thanks

[1] The "continue" at https://git.launchpad.net/content-cache-charm/tree/reactive/content_cache.py?id=2d4a14b89bcc54f23f067b95a5dd53f084f6ff46#n320 is before "backend-maxconn" gets considered at https://git.launchpad.net/content-cache-charm/tree/reactive/content_cache.py?id=2d4a14b89bcc54f23f067b95a5dd53f084f6ff46#n341 so the default value from https://git.launchpad.net/content-cache-charm/tree/lib/haproxy.py?id=2d4a14b89bcc54f23f067b95a5dd53f084f6ff46#n261 is used.