installer: Fix password page

Due to the cmigration to the new ConfirmedEntry widget, it was missed to
store the password in the password page model. This is now fixed and the
password is stored in the password page model.

Change-Id: I290c69683f6a2a1126533befb4f35b816406ed42
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1007811
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>