(issue#407) Enable authn_passthrough for Cobbler Web

There is currently no support for kerberos single sign-on in cobbler web.
This patch allows apache or another web server to set the REMOTE_USER variable which
will be used as the authenticated user for cobbler API authorization calls
Merge pull request #408 from jjulien/issue407

(issue#407) Enable authn_passthrough for Cobbler Web