Add container support, e.g. singularity

[christopheredsall]

It would be convenient to be able to run containers on the compute nodes, in particular it would be good to support Singularity as it fits the model of an HPC cluster - the containers run as a cluster local user and the MPI inside the containter can talk to the MPI in the host.

I have a proof of concept which makes use of a hook to pull in an ansible playbook (#42) on the management node which is a nicely generalisable solution, but for this specific case it would be good to have it in the base system.

(Maybe at a later point we could consider Podman if that is the way the HPC community goes)

[milliams]

This is now easily doable using the custom compute image hooks. I will leave this issue open until we have documented this as a supported workflow though.