(MAINT) Update changelog to KAC standard
(MAINT) Update changelog to KAC standard

Prior to this commit the `puppetlabs-reboot` project used
a custom changelog format. This maintenance commit updates
the changelog to follow the KeepAChangelog standard and
adds the unreleased section to the top of the changelog.

puppetlabs/puppetlabs-reboot,puppetlabs/puppetlabs-reboot