sysusers: read passwords from the credentials logic

sysusers: read passwords from the credentials logic

Let's make use of our own credentials infrastructure in our tools: let's
hook up systemd-sysusers with the credentials logic, so that the root
password can be provisioned this way. This is really useful when working
with stateless systems, in particular nspawn's "--volatile=yes" switch,
as this works now:

 # systemd-nspawn -i foo.raw --volatile=yes --set-credential=passwd.plaintext-password:foo

For the first time we have a nice, non-interactive way to provision the
root password for a fully stateless system from the container manager.
Yay!