r/squid: Support custom cache rules

Instead of hard-coding a single cache directory and a set of refresh
patterns, the *squid* role can now have custom cache rules defined with
the `squid_cache_dir` variable (which now takes a list of `cache_dir`
settings) and the `squid_refresh_pattern` variable (which takes a list
of refresh patterns).  If neither of these are defined, the default
configuration is used.

This is a breaking change, since `squid_cache_dir` used to refer to a
directory, and the previous default was to configure one cache path.
There are no extant users of this role, though, so it doesn't really
matter.

roles/squid/handlers/main.yml

@@ -1,3 +1,7 @@
+- name: initialize squid cache directories
+  command:
+    /usr/sbin/squid -N -z -F -f /etc/squid/squid.conf
+
 - name: reload squid
   service:
     name=squid