Remove setting of DEVSTACK_GATE_EXERCISES
Remove setting of DEVSTACK_GATE_EXERCISES

This defaults to 0 anyway, and is being removed as the devstack
excercises are being removed. see https://review.openstack.org/#/c/581377/

Change-Id: I80b172b85fa29ae71cf3e14d63d654a5fea56c0c

openstack/heat-templates,openstack/heat-templates,openstack/heat-templates