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 know you're following the style of the rest of the code, but for future reference, I think this style of code is ugly and awful and we should just write it out "longhand"
thusly:
writeFileAtomic(h.syslogConfig.LogrotateConfPath(), logrotateConfFile, 0600, syslogUid, syslogGid)