Fix nompi configuration

Missed a communicator argument in the mesh_communication.C implementation for the no mpi case.

Also, for --disable-mpi, we define COMM_WORLD as an int in libmesh_common.h, where for chicken-and-egg reasons it is difficult to include all of parallel.h. Correspondingly, later things freak when trying to construct a Parallel::communicator from an int, as that constructor is not defined. This implements that constructor.