Install HWE kernels

Install `hwe-support-status` which returns non-zero exit code if no HWE
kernel updates are available.

Download and install the latest kernel as set by they hwe_ver Hiera key if
`hwe-support-status` exits with a non-zero exit code.
Add supported_kernel keys to Hiera

- `::enabled` is used to feature-flag HWE upgrades (default: false)
- `::hwe_ver` is used to specify the HWE version in Hiera (default: none)
Document class