Bug 895639: Add preliminary support for OS X 10.9; r=armenzg

This requires that fixes for
  * http://projects.puppetlabs.com/issues/21868
  * http://projects.puppetlabs.com/issues/22107
  * http://projects.puppetlabs.com/issues/22129
be in place.

Note that this uses the built-in user provider, rather than the custom
darwinuser provider.  Other OS X versions will switch soon.

This is "preliminary" because we may need to adjust as we begin running
tests on these hosts.