tests: Use testrunner.TEMPDIR

Storage functional tests used to define their own temporary directory
path. They use now the testrunner default TEMPDIR which suits also the
storage tests.

Change-Id: Ib3a8047ef0f5d4fdccf79b5000b5385f371f4db8
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27503
Reviewed-by: Vered Volansky <vvolansk@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>