Fix a few nits in docker-rally.sh
Fix a few nits in docker-rally.sh

Need to re-set xtrace after prepare.sh and rather than
return with the status, we need to exit with it.

Signed-off-by: Ryan Moats <7ac70bcc48c94124ee1b01a4779bf5a8a6faebbd@us.ibm.com>

sivakom/ovn-scale-test,openvswitch/ovn-scale-test,sivakom/ovn-scale-test,openvswitch/ovn-scale-test