Add SSH username and public key to Hiera

This commit adds the SSH username and public key to Hiera so that they can be called from other locations, and updated only once.
Re-works YAML, adds SSH username and public key

Adds:
- SSH username
- SSH public key

Moves key/value pairs around in order to make the file neater.
Add authorized keys
Adds relevant .gitkeep placeholders

This commit adds .gitkeep placeholders ensuring we don't lose folder structure.
Add base::user, rename ssh-key key
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.
Adds gds_account k/v pairs to Hiera

This commit:
- adds gds_account::group key/value pairs to Hiera
- adds gds_account usernames and ssh keys to Hiera
Amends Hiera ssh-key name
Adds GOV.UK Backup User to Hiera

This commit adds the GOV.UK Backup User to Hiera such that the creation of the
user with the correct SSH key, correct shell (rssh) and correct groups are
created in Hiera. This removes the need for a separate user class, as Hiera can
handle the user in the same way it handles other users through the gds_accounts
module.
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.
Add/remove manifests, set create_group=true

Ready for a rebase fixup at a later stage
Fix syntax, add rssh
Re-adds purge_ignore