force encode all strings to UTF-8

- this is a bit of a hack for ohai-6 while we're still using
  popen4 instead of mixlib-shellout
- this may mean that we poorly support ISO-8851 / Shift_JIS or other
  non-UTF-8 encodings for characters where there is no
  translation between the external encoding and UTF-8
- this essentially standardizes on using UTF-8 internally