add option to exclude files/dirs from rsync

[troyready]

This allows us to keep some files in the test-kitchen project from being transferred into the instance.

Use by specifying an array under :rsync_options=> :excludes, eg:

transport:
  name: rsync
  rsync_options:
    excludes:
      - foobigfiles
      - bar/other*

I'm sure my use of a global variable to pass the data is about the worst thing ever, but I couldn't figure out a better way to get it into the Connection class. Open to suggestions on any of it, and happy to update the documentation too if I can get this in an acceptable/merge-able state.