(PUP-2660) user: set the user should-value when generating ssh_authorized_keys

When purging authorized SSH keys, the user type generates ssh_authorized_keys
resources using eval_generate. The ensure property receives a should value of
"absent" so the keys are removed. However, those resources are non-functional
because the user property lacks a should value. The resource fails during
Provider::Ssh_authorized_keys::Parsed#flush.

Fixed by setting the appropriate user value for each generated key.