Etcd for RHEL and CentOS in openstackinstallguide

[Eric]

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [x] This doc is inaccurate in this way: __no mention on which node etcd should be installed.____
- [ ] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

If you have a troubleshooting or support issue, use the following resources:

 - Ask OpenStack: http://ask.openstack.org
 - The mailing list: http://lists.openstack.org
 - IRC: 'openstack' channel on Freenode

-----------------------------------
Release: on 2019-01-17 07:22:07.431776
SHA: 1c0bbd7a78c55a0dee3560007f4125e1d4c6f2ca
Source: https://git.openstack.org/cgit/openstack/openstack-manuals/tree/doc/install-guide/source/environment-etcd-rdo.rst
URL: https://docs.openstack.org/install-guide/environment-etcd-rdo.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/632941