Add devstack-gate repo to zuul v3
In order for us to be able to hack on devstack-gate jobs, we need to
have devstack-gate be in zuul.
# After this point, sorting projects alphabetically will help
# merge conflicts
- openstack-dev/sandbox
+ - openstack-infra/devstack-gate
- openstack-infra/nodepool
- openstack-infra/shade
- openstack-infra/zuul
