We are starting to use the apache2 mod_cache_disk functionality more now
and during use the cache has grown into the 1.5GB range. The
htcacheclean process is cleaning up every 2 hours which is how it is
getting behind with its limit of 300MB. Reduce the interval to 15
minutes by supplying an /etc/default/apache-htcacheclean config.
Note we cache status.json files which are only valid for a very short
period of time. This likely explains the quick growth of the cache.
Change-Id: Iff00fb1806796ef6db26e53e026c533c47a902b4
608f56ab82