Add alphagov/gds_accounts support

This commit includes support for 'alphagov/gds_accounts', which is a Puppet
wrapper for managing access by GDS staff to projects over SSH.
Add Hiera key/value pairs

This commit:

- Adds the `gds_accounts` and `base::packages` classes to Hiera, ready for
  autoloading as we are using Hiera as a lightweight ENC in this repository

- Adds two groups to `gds_accounts` in order that named users have the ability
  to access the machine via SSH and administer it. Belonging to the correct
  groups is a prerequisite to allow this to happen.

- Adds named users using `gds_accounts`, specifying username, comment (full
  name) and public SSH key. These values are consistent across all
  repositories making use of the `gds_accounts` module.

- Adds a specific `govuk_backup` user using `gds_accounts`, specifying a comment
  for easy identification at a later stage, a default shell (rssh, in order to
  restrict the permissions the user has to run programs, a `true` boolean value as
  the value for the `create_group` key in order to both ensure that the user
  does not get automatically added to a primary group with permissions above the
  level it should have, and to ensure that the user is jailboxed within its own
  `govuk_backup` group, and an empty array for the value of the `groups` key in
  order to ensure the user is not added to any secondary groups.
Separate purge_ignore list in to new Hiera YAML

This commit separates the gds_accounts::purge_ignore list out to a separate
Hiera YAML file in order that we can properly account for and track the creation
of these accounts using Puppet.