Ensure Linux/ppc64le OSUOSL machines are running a sensible set of OSs

[sxa]

Current list of test machines from the inventory:

          centos74-ppc64le-1: {ip: 140.211.168.228, user: centos}
          centos74-ppc64le-2: {ip: 140.211.168.217, user: centos}
          ubuntu1604-ppc64le-1: {ip: 140.211.168.227, user: ubuntu}
          ubuntu1604-ppc64le-2: {ip: 140.211.168.190, user: ubuntu}
          ubuntu1804-ppc64le-1: {ip: 140.211.168.5, user: ubuntu}
          ubuntu1804-ppc64le-2: {ip: 140.211.168.8, user: ubuntu}
          ubuntu2004-ppc64le-1: {ip: 140.211.168.235, user: ubuntu}
          ubuntu2404-aarch64-1: {ip: 140.211.169.12, user: ubuntu}

We should look at getting rid of the Ubuntu 16.04 and 18.04 machines and replacing them with more up-to-date ones. We should also consider whether it is worth having something newer in place for the CentOS ones. We should put together a list of what OSUOSL can provision on these machines and decide on what to allocate accordingly.

We also have the following build machines:

          centos74-ppc64le-1: {ip: 140.211.168.138}
          centos74-ppc64le-2: {ip: 140.211.168.117}

which as far as I'm aware are no longer used as we are building in docker containers, so their resources can likely be reallocated to test (or another dockerhost) too.

[sxa]

Related: #3588 (comment)

[sxa]

@Haroon-Khel Can you confirm that you are able to access the OSUOSL OpenStack console?

[sxa]

@Haroon-Khel Since you were working on provisioning new ones recently, can you close this off with a comment including the list of systems/distributions we now have for Linux/ppc64le please?