Documentation: add OpenStack to integrations.md
- [Vitess](http://vitess.io/) - Vitess is a database clustering system for horizontal scaling of MySQL.
- [lclarkmichalek/etcdhcp](https://github.com/lclarkmichalek/etcdhcp) - DHCP server that uses etcd for persistence and coordination.
- [openstack/networking-vpp](https://github.com/openstack/networking-vpp) - A networking driver that programs the [FD.io VPP dataplane](https://wiki.fd.io/view/VPP) to provide [OpenStack](https://www.openstack.org/) cloud virtual networking
+- [openstack](https://github.com/openstack/governance/blob/master/reference/base-services.rst) - OpenStack services can rely on etcd as a base service.
