General refactor / cleanup / housekeeping

* Added test for ntp.conf contents to verify_installed.bats.
* Cleaned ntp.conf.erb for readability & indentation.
* Restricted the number of servers & peers that will ever be written to the ntp.conf file to 7, in case array is merged to too many entries.
* 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.