Missing steps for ubuntu 20.04 cloud-archive for Ussuri

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:

add-apt-repository cloud-archive:ussuri doesn't work on Ubuntu 20.04(focal) whereas as per openstack, focal release is supported for ussuri installation. The issue is that the cloud-archive doesn't have the required dists for focal-proposed or focal-updates with Ussuri on the mirrors.

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

-----------------------------------
Release: on 2020-05-28 17:21:26
SHA: 1c4ab4c62a31fc3b222e8736af8bfb1fff0b3c21
Source: https://opendev.org/openstack/openstack-manuals/src/doc/install-guide/source/environment-packages-ubuntu.rst
URL: https://docs.openstack.org/install-guide/environment-packages-ubuntu.html