Fixed issue 3918

Running Windows guest commands through a scheduled task were not returning the correct exit codes, they were only returning 1 or 0. This has negative consequences especially for Puppet which can return an exit code of 2 for partial success.

Since we're running an executable from inside a powershell encoded command we need to ensure we explicitly propagate the exit code to the original caller just like a regular PowerShell script - in this case cmd /c which in return is called from a scheduled task.
Merge pull request #3922 from mitchellh/fix-issue-3918

communicator/winrm: proper exit code for powershell
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