win_system: Fix python 3 string decode issues
win_system: Fix python 3 string decode issues

Python 3 no long has a decode function for strings.
Decode should not be called if the computer description
or shutdown message are already strings.

Signed-off-by: Molly Kreis <2a825f58bb965f2fb430115b7516a58d75507a90@ni.com>

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt