rsyslog: configure and setup log rotation for all-machines.log

Implements sized based rotation for the all-machines.log.

See: https://bugs.launchpad.net/juju-core/+bug/1078213
I guess yes, I could just write return err there, but I don't think that simplification makes the code more readable. I think it is good to be clear that the else block is only returning an error that is NOT of type NotExist, just like the block above it. Perhaps I should make a helper function to encapsulate the Stat / IsNotExist logic for both files?