fix tox-gate.sh exit code
fix tox-gate.sh exit code

Previous version of the gate was failing to return
an error code if tox execution failed.

Now it uses bash strict mode which should return the
right exit code.

Change-Id: I7a597cb888e1c7836128453042135586cc12b521

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared