Use explicit defaults for NTP checks

Using the defaults as in https://github.com/sensu/sensu-community-plugins/blob/c6cef387d980e17ba4e493f9cd48671c3b66ecf7/plugins/system/check-ntp.rb
but having the explicitly here so that people don’t have to go looking
elsewhere for the values.

I’ve asked in the #sensu channel on freenode for more documentation
about the the check-ntp plugin, but for now, this should quieten our
alerts and we can tune them in the future.