Mod_proxy_cluster: mod_proxy_cluster unauthorized mcmp requests

A vulnerability was found in mod_proxy_cluster. The issue is that the <Directory> directive should be replaced by the <Location> directive as the former does not restrict IP/host access as `Require ip IP_ADDRESS` would suggest. This means that anyone with access to the host might send MCMP requests that may result in adding/removing/updating nodes for the balancing. However, this host should not be accessible to the public network as it does not serve the general traffic.