installer: Remove eventual password variables

A greedy approach to make sure that no passwords are kept in the central
configuration file.

Change-Id: I39c8af11b633e17902cd3b98fcea4606cad0edd0
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1062541
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>
(cherry picked from commit 3f867a652ff0d3df12df52f3958b736cbc597b73)