Add user manifest

This commit adds a user manifest and pulls the username and public key in from Hiera. It limits the user to the rSSH shell in order such that the user can only use rsync, and nothing else. This is a security measure.
Add authorized keys
Adds relevant .gitkeep placeholders

This commit adds .gitkeep placeholders ensuring we don't lose folder structure.
Ensures user is present
Merge branch 'add-hiera' of github.com:alphagov/govuk_offsitebackups-puppet into add-hiera
De-hieraises username

@dcarley noted that if the username is a variable, with the value set in Hiera,
then if someone changes the username in future, any file ownerships will not
change, and the old account will not be removed. This type of functionality is
outside of the remit of the initial scope of this repo, so given that the
username will not change, it is easiest to include the username directly within
this file.

The SSH key, however, is set in Hiera as this may change in future, and changing
it in Hiera should ensure Puppet changes the value where required on the box at
the next Puppet run.