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

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.

Start by reviewing the current test and build machine inventory listed in the issue, then determine which operating systems OSUOSL can provision for the ppc64le machines. Done means documenting the available options, agreeing on an updated test allocation, and deciding whether the unused CentOS build machines should be reallocated.

Written by the indexing model from the issue text.