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
this is testing the file exists, you can use os.Stat for that, or even os.Open. It may be useful to add a helper

AssertPathExists(c, path) 

or something.