Ensure HWE support

- Allow `$hwe_ver` to be passed from Hiera as a class parameter
- Install Linux kernel images and headers for $hwe_ver

Doing this keeps the HWE packages installed going forward, so we should
never be in a situation where Puppet isn't managing HWE support for us.
Make /etc/sudoers match other environments

The sudoers file in this environment doesn't match the sudoers file we have
in other Puppetised environments. Specifically, the `setenv` and `env_reset`
defaults have been added to ensure that the user can preserve environment
variables, if required.