Add `matrix: include` to multispec

[frenzymadness]

We use distgen in s2i Python container and the matrix: exclude part of our multispec.yml is growing so it might be better for us to specify the opposite – include instead of exclude, which would allow us to say what exact combinations we want.

I might try to implement this to distgen. What do you think about it?

[praiskup]

I'm +1, but we should probably ask @zmiklank?

[zmiklank]

If this change would be backward compatible - preserving the matrix: exclude, I am also +1.