minor changes in passwd.py

Replace Exception with RuntimeError.

Change-Id: I4ced7980e975e2cce588261812ecdb1847b5941f
Signed-off-by: Mooli Tayer <mtayer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25489
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>