Removing mocking of _start function and add extraCmd variable to proxy

Instead of adding monkeyPatch only for adding PYTHONPATH before running
the command, setting self.extraCmd will append extra field to the
execution.

Change-Id: I790fb1366bcfd369bd6be1618c284f2a945c19ca
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14585
Reviewed-by: Dan Kenigsberg <danken@redhat.com>