Allow using get_dsl_diffs mutliple times for haproxy
When using get_dsl_diffs multiple times for haproxy, the first call would correctly compute the acl patterns, but subsequent calls would output no patterns at all. This was due to the fact that the patterns registry wasn't being reset between calls.
This bug of course doesn't show up in the CLI, as we never call dsl diffs for haproxy multiple times from the same api class, but it showed up in hiddenparma where we do.