Allow disabling of SSL peer verification in knife-windows for testing

After some discussion, it seems to me that management of ssl communication over winrm is at a significantly different knowledge level compared to management of ssl against public Internet resources (e.g. RubyGems, Hosted Chef, S3, etc.), or on-premises resources (e.g. a Chef Server) managed by `ssl_verify_mode`. So it's not unlikely to me that you'd be happy with the default (in Chef 12) of `:verify_peer` for those scenarios, but not for your winrm use cases. So this lets you manage it separately.
This does not preclude us from having `ssl_verify_mode` become the default in the future for this setting or otherwise doing away with the separate management scenario.