Enable HWE support

The GOV.UK mirrors run Ubuntu 12.04 (Precise), for which the kernel has now EOL'd. Ubuntu's [workaround](https://wiki.ubuntu.com/1204_HWE_EOL) for this allows hardware support to be brought forward from a later release (in this case, 14.04 (Trusty)), and thus make the installed kernel supported for a longer period of time.

At some stage, we might want to look at upgrading these to Trusty.
Hmm. Should we remove this, and just set the `if` below to rely on being true (which will be the case if there's a value passed in from Hiera for `hwe_ver`)?