Install Trusty HWE kernel

The version of the Ubuntu kernel we're currently using EOL'd on August 4th.
This means that whilst we might still receive updates for software installed
on the machine, we won't receive any security updates etc. for the kernel,
which is a bad thing.

To combat this, this commit installs the Trusty HWE kernel. Initially, it
installs it to a mirror][ to check this works properly when using Puppet
(it worked fine when using Apt in no-op mode).
Allow vagrant user to preserve the environment

Vagrant derives it's privileges by membership of the %sudo group.
However the defaults for sudo don't include env_reset (which executes
commands in a minimal shell environment by default) or setenv (which
allows users to override env_reset if neccessary), which causes
reprovisioning a Vagrant machine to fail.

We should stop doing that.
Merge pull request #37 from alphagov/allow_sudo_setenv

Allow vagrant user to preserve the environment
Merge branch 'master' of github.com:alphagov/govuk_mirror-puppet
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