Provide base images that work with the v2 operator

[alculquicondor]

We need a base image that has OpenSSH properly configured. The other example images should inherit from it.

We can also provide a base image that contains OpenMPI and another for Intel.

[alculquicondor]

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[alculquicondor]

I proposed some base images in #419, but I couldn't migrate the horovod sample to it. The horovod sample crashes. My guess is that it's either because of a different OpenMPI version or Python version.
But I'm not familiar with horovod, so I would appreciate if somebody else can take a look.
The existing image based on ubuntu works.