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
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.