[COOK-3651] Refactor and standardize cookbook

- Added test for ntp.conf contents to verify_installed.bats.
- Cleaned ntp.conf.erb for readability & indentation.
- Refactored the per-platform settings in the default attributes file to minimize the number of attribute changes.
- Switched per-platform attribute case statement to use platform_family. As part of this change, switched default service name string to 'ntpd' instead of 'ntp', updated unit tests accordingly.
- Ensured that Linux hosts would not try to run the windows_client.rb recipe.
- Ensured that Windows hosts would not try to run the undo.rb recipe.
- Updated the README.md for completeness & clarity.
- Renamed CONTRIBUTING to CONTRIBUTING.md.
- Quick update to metadata.rb to add xcp (RHEL variant) to supported OS list. See GH pull request #14.

Signed-off-by: Seth Vargo <sethvargo@gmail.com>