.fmf/config define type/flavor of an fmf tree

[LecrisUT]

Let's say we want to enforce /plans and all of its sub-trees to be of a tmt-plan type, e.g. it should be checked against the json schema of that type, etc. This type could be defined in the fmf config. If we also allow to overwrite the configs in the same logic as the fmf tree, this could be rather powerful organization scheme