Add blom/rssh module to Puppetfile

rssh (http://www.pizzashack.org/rssh) is a restricted shell for use with
OpenSSH. It allows us to lock down which programs can be run by which users from
the shell.

This commit adds the rSSH module in to the Puppetfile.
Allows rsync in rssh

The rssh Puppet module was added in be7e918, so this commit adds an rssh.pp
manifest in order to configure rssh.

This commit adds an `allow` attribute to the rssh class in order to specify that
only rsync should be allowed. Full details on the values the `allow` attribute
can accept are at https://forge.puppetlabs.com/blom/rssh.
Sets default shell by adding user.pp manifest

This commit sets the `govuk-backup` user's default shell to `rssh` in order to
restrict the user's permissions to run programs from shell. This has the effect
of limiting the power of the `govuk-backup` user for added security.