[bugfix] Treat correctly Slurm constrains with hyphens when filtering nodes

[vkarak]

When converting the constraint expression to a Python one we replace hyphens in constraints and node features with _ and then evaluate the validity.

Ideally, we would need to properly support the Slurm constraint syntax, but that would an enhancements on its own.

Closes #3325.

[victorusu]

lgmt. I wonder how we haven't caught this before. Should we also add unit tests?

[vkarak]

I wonder how we haven't caught this before. Should we also add unit tests?

The problem is that we can't emulate the flexible allocation easily for a unit test. Generally our solution is suboptimal, since we don't recognize the full syntax of Slurm constraints. We simply provide a minimal meaningful subset, because I don't know if it makes sense to re-implement Slurm's node selection mechanism by constraints. Ideally, Slurm should be giving us a way to do the node selection outside submission.