log rotation

Partial fix for bug https://bugs.launchpad.net/ubuntu/+source/juju/+bug/1078213

This implements a rotating log from inside the code and removes the upstart redirect for new deployments.  For existing deployments, the upstart redirect will still exist, but nothing will get written to stderr, so it won't do anything.  This seems sufficient, and less error prone than trying to update the upstart script of existing deployments for essentially no reason.

This only rotates the machine-n.log and unit-n.log files, it does not rotate the all-machines.log, which requires a change to rsyslog, which I figured would be fine as a separate commit.