hosted-engine: don't log the whole backtrace for expected problems

when deploying host for hosted engine, vdsm starts using the HE
broker api before the deployment is finished, and the required config
file is not writen, so it results in the exception which polutes the
log

Change-Id: Ib03fbe80dfd111ef9a2e4f2137433e358f6479bb
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1045053
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31432
Reviewed-by: Sandro Bonazzola <sbonazzo@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>