[chef] Secure knife winrm

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Trying to get knife winrm working with SSL enabled.  Keep getting error with
winrm validating the servers WinRM certificate.  Error: unable to get local
issuer certificate.

I am relativly sure i need to provide a certificate chain but attempts at
providing ca-trust-file have faild.  Can anyone provide link to documentation
on how format and content needed for this file.  

Thanks
Daniel D.