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
I decided to default to the current behavior if the os is unknown (the only error GetOSFromSeries will return). We can panic if you believe this is the best course of action.