disable cache purging if caching is not enabled

[agitator]

with unreleased version from master

Cache purging should be disabled if caching is not enabled or not enabled with caching proxy like varnish to prevent messages like this:
ConnectionError: HTTPConnectionPool(host='localhost', port=6081): Max retries exceeded with url: ... (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x10f3ce110>: Failed to establish a new connection: [Errno 61] Connection refused',))