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
You are right, this can be a private variable.I would rather have it as a variable in case we need to overwrite it later for testing. We need to get the tests working on windows first though.