Add govuk-assets SSH key
Add govuk-assets user

In order to ensure that off-site backups of our assets aren't accessible
should the govuk-backup user become compromised, create a govuk-assets user
to own the assets files and folders. Ensure that its default shell is rSSH
to restrict attempts over any protocol other than those already defined in
the rSSH configuration.
Ensure /srv/backup-assets is owned by correct user

Previously, /srv/backup-assets was owned by the govuk-backup user, but
Duplicity can't mkdir directories within when connecting as the govuk-asset
user in this instance.

Ensure /srv/backup-assets is actually owned by govuk-assets to get around
this problem.
Merge pull request #67 from alphagov/create-govuk-assets-user

Create govuk-assets user