Deactivate workers for windows

Authenticationworker and rsyslog workers are not supported on windows yet. The authentication worker will be ported on windows to add support for winrm. Rsyslog is not supported on windows yet.

This pull request depends on https://github.com/juju/juju/pull/244
It seems like we need a spot for "Current OS" so we don't have to keep throwing away the error.  Get it once on startup and put it somewhere public.