Introduce MPI communicator

[rlnx]

• [x] Create spmd_communicator_via_host_impl base class for communicators
• [x] Add MPI dependencies to Bazel
• [x] Create mpi_communicator
• [x] Create tests for mpi_communicator
• [x] Create infrastructure in Bazel for writing MPI integration tests
• [ ] Create MPI integration tests for K-Means

[napetrov]

@amgrigoriev - guess we don't need this PR and it can be deleted?