[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>
COOK-3636 Allow ntp cookbook to update clock to ntp servers

Adds two new boolean attributes:
node['ntp']['sync_clock'] - When true, this attribute forces the ntp daemon to halt, executes ntpd -q, and then starts the ntp daemon again. This will repeat every Chef run while the boolean remains true.

node['ntp']['sync_hw_clock'] - When true, this attribute runs the command hwclock --systohc on supported operating systems, forcing the hardware clock to sync to the system time. This will repeat every Chef run while the boolean remains true.
COOK-3636 Added tests & clarified README.

COOK-3636 Added tests for new attributes & clarified README.
Update metadata.rb formatting
Version bump to v1.5.0
Version bump to v1.5.1
Add fix for ntp.leapseconds apparmor error
Fixes [COOK-3023]
Set apparmor to restart on notify
Fix spec test for apparmor recipe
Requested fixes
Fix documentation of new behavior
Fixups for Travis
Implement requested changes.

* Moved attribute documentation to relevant part of README.md
* Added two more chefspec tests
* Fixed a reversion caught by the new chefspec tests.