Fix a typo in variable name

This variable should be called DEVSTACK_GATE_TEMPEST_NOVA_V3

https://github.com/openstack-infra/devstack-gate/blob/master/devstack-vm-gate.sh#L230

Change-Id: I890c1e6f1ad41320df8b4736ad84c03176e0d8a6
Merge "Fix a typo in variable name"