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.
Merge pull request #38 from alphagov/trusty-hwe

Enable HWE support