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.
The message for this commit says:

> Specifically, the `setenv` and `env_reset` defaults have been added to ensure that the user can preserve environment variables, if required.

But these were already set. Should this commit be in this PR? Do we need to make any more changes prior to switching it to a different sudo module in [#77440542](https://www.pivotaltracker.com/story/show/77440542)?