juju should log at environment URL

I'm adding support for the API server to expose environment-specific URLs for things like '/logs' and '/charms'. For compatibility with old servers we still directly log to '/log', though. When we find a good way to determine the API server supports the new URLs, we should use them.

I believe William's request was that we do a regular API call which returns the URL that we should connect to to get logs/upload charms/etc. It adds a round trip, but the round trip overhead vs the time spent to actually upload the data should be lost in the noise and makes us more resilient if we want to change things in the future.