Making logging to MongoDB the default

[Menno Smits]

I implemented the ability for Juju's logs to go to MongoDB some time ago.
This feature is available in 1.25 behind a feature flag and also gets
enabled automatically if the "jes" feature flag is enabled (logging via
rsyslog doesn't make much sense when using multiple environments in the one
system).

There's also another separate feature flag which turns off rsyslog based
logging.

The "jes" feature flag will be removed soon meaning that database logging
will need to become default functionality. The question is do we then also
remove logging to rsyslog functionality, or perhaps reverse the sense of
the rsyslog feature flag so that it's off by default but can be enabled if
people want it for some reason?

I'm confident that the logging to the database feature is solid. I spent of
lot of time confirming that performance wouldn't be an issue. The code is
well tested. Automatic log rotation is implemented.

The main issue I can see is that once rsyslog based logging is turned off
we lose the all-machines.log file which some people and systems no doubt
rely on. The logs for an environment can of course still be retrieved using
the "juju debug-log" command.

If we really want something like all-machines.log, it wouldn't be /too/
hard to implement a worker which generates an all-machines.log style file
in real-time from the database logs. All the pieces to implement that
already exist, but I really don't have the bandwidth this cycle to do the
work.

Thoughts?

- Menno
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