Add Spack example

[heyealex]

Adds a spack specific example that creates a build VM for Spack that
shuts down on completion. Includes a workstation VM for testing and
verifying the spack installation.

Also made minor updates to the other examples.

Submission Checklist:

• Have you installed and run this change against pre-commit? pre-commit install
• Are all tests passing? make tests
• If applicable, have you written additional unit tests to cover this
  change?
• Is unit test coverage still above 80%?
• Have you updated any application documentation such as READMEs and user
  guides?
• Have you followed the guidelines in our Contributing document?

[cboneti]

a few minor comments

[heyealex]

I ran into an issue with the nfs-server while testing where the spack directory under /apps would be created before the spack builder could get to it.

I'm still investigating why this is happening. This example works using filestore, but before I merge this, I want to verify the problem was with nfs-server or elsewhere and not spack-install.

[heyealex]

I've not been able to replicate the bug from yesterday. I'm guessing it was some issue with my environment that I'll continue to monitor, but this PR should be ready to be re-reviewed now.

[douglasjacobsen]

From the spack point of view, I think this LGTM. :)

[nick-stroud]

So much prettier! 😍

[douglasjacobsen]

@heyealex and I talked about this offline, but something that feels fragile to me is having the dependencies outside of the install-spack resource. This allows users to select the install-spack resource, but not the dependencies, which would cause the install-spack resource to fail in a non-obvious way.

However, there are other features that are going to be developed within the toolkit eventually that will make this easier to deal with. So I'll approve this for now with the assumption that it might be reworked in the future.