Make environment:mpi:gpu optional

[albestro]

This PR:

• extends tests with a basic test-case with an environment where mpi:gpu field is not specified
• fix schema to have a default for the mpi:gpu field (default = null)
• fix validator to deal with non-existing sections

About this latter point, since I don't have any experience with this library, I'd like to ask you to double check (see comment in the code) if it is the correct reason/solution.

Reference

According to the documentation the mpi:gpu field is optional, but the schema and validator were not supporting that.

stackinator/docs/recipes.md

Lines 141 to 147 in e8219f5

	To configure MPI without GPU support, set the `spec` field with an optional version:
	```yaml title="environments.yaml: MPI without GPU support"
	host-env:
	mpi:
	spec: [email protected]
	# ...
	```

stackinator/docs/recipes.md

Lines 149 to 161 in e8219f5

	GPU-aware MPI can be configured by setting the optional `gpu` field to specify whether to support `cuda` or `rocm` GPUs:
	```yaml title="environments.yaml: GPU aware MPI"
	cuda-env:
	mpi:
	spec: cray-mpich
	gpu: cuda
	# ...
	rocm-env:
	mpi:
	spec: cray-mpich
	gpu: rocm
	# ...
	```

[bcumming]

Thanks @albestro, this was an error and your solution is sound!