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
Can you write a quick unit test for this, just simple "make sure it does what we expect it to"?