Merge lp://qastaging/~cjwatson/charm-haproxy/listener-address-localhost into lp://qastaging/charm-haproxy

Set statistics listener-address to 127.0.0.1.

If `listener-address` isn't set on the relation, the telegraf charm defaults to using the unit's `private-address`; but `monitoring_allowed_cidr` defaults to `127.0.0.1/32`, so if telegraf tries to talk to haproxy on the unit's private address then it will get a 403 error unless `monitoring_allowed_cidr` has been specifically configured to allow the unit's private address. Doing that is inconvenient in bundles that are to be deployed in multiple network environments.

The statistics relation has `scope: container`, so the other end of it is always going to be on the same machine, and it makes sense to just advertise 127.0.0.1. This matches the behaviour of the content-cache charm.