Other nodes in Installation Guide

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:

- [ ] This doc is inaccurate in this way: ______
- [x] This is a doc addition request.
- [x] I have a fix to the document that I can paste below including example: input and output.

On RHEL and CentOS 8 if firewall is blocked on controller node, then the clients cannot time synch with it. Update firewall rules as follows

# firewall-cmd --permanent --add-service=ntp
success
# firewall-cmd --reload
success
#

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

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

-----------------------------------
Release: on 2020-08-04 14:52:09
SHA: d7e0852e8e5d75e8e6203305e328b07ea49d32ef
Source: https://opendev.org/openstack/openstack-manuals/src/doc/install-guide/source/environment-ntp-other.rst
URL: https://docs.openstack.org/install-guide/environment-ntp-other.html