use SASL_USERNAME instead of its upstream value

commit 98a2692fe reverted broken code, but it also added new code, that
the reviewer failed to notice. This patch fixes it.

Bug-Url : https://bugzilla.redhat.com/show_bug.cgi?id=1168689
Change-Id: Idf7800e26d95e5da9d64b615de420389bbc7e52f
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35799
Reviewed-by: Martin Sivák <msivak@redhat.com>