engine.py: replace sed to setVdsConf

Use the standard way to write into vdsm-reg.conf.

Change-Id: I16a0889843feba29abbfb02f3d81293df7f2f3fc
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=890572
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12299
Reviewed-by: Joey Boggs <jboggs@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>