MPI: Handle sanity checks during initialization of unstructured boundary conditions differently

[lchristm]

During initialization of the boundary conditions for unstructured meshes (UnstructuredMesh and P4estMesh), there is a check if the boundary names given by the user via a dictionary are valid. This check compares the names given by the user in the dictionary to the names found in the boundary container. While this works fine for serial runs, this check can't be done in parallel runs because the local domain might not contain all boundaries and thus not all valid boundary names are present in the boundary container. In that case the check would fail even if all boundary names given by the user are valid.