infra: Rearrange PYTHONPATH for remoteFileHandler

When running tests P_VDSM is set to $GIT_ROOT which makes "import vdsm"
conflict. This makes sure we only take from the root dir in case there
is no other alternative.

Change-Id: Id4eed6fb94686d4e77a23e0d3b6505bb4bd439cd
Signed-off-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14627
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>