[Slaves - Windows] Fixes a bug with Windows slaves.

On Windows, when slave.jar changes on the master, the
'slave_jar_resource' remote_file will try to replace the file.
This means we get 'Errno::EACCES: remote_file[C:\chef\cache/slave.jar]
(dynamically defined) had an error: Errno::EACCES: Permission denied'
error since the file is currently in use (java -jar slave.jar is
running). Setting atomic_update to false means we will just use cp,
which works.