synced_folders/rsync: add UserKnownHostsFile to /dev/null [GH-3511]
Backfilled unit tests for Windows guest support

- Fixed typo in helper test
- Removed extraneous machine.config prefix from Windows guest config validation
- Added WinRM communicator unit tests
- Added Windows guest capability unit tests
Added Windows guest file manager for recursive uploads

- Since WinRM uploads are so slow, MD5 content checking is used
Add WinRM file transfer error and english text
Merge pull request #3516 from mitchellh/backfill-windows-guest-unit-tests

Backfilled unit tests for Windows guest support
Merge pull request #3519 from mitchellh/windows-guest-file-manager

communicators/winrm: Windows guest file manager