Get infiniband working with openmpi
The openmpi expression and ibverbs expression need to be modified as appropriate to get OpenMPI working over InfiniBand on the clusters. I expect this will involve modifying ibverbs expression to hook into the appropriate device-specific user-space libraries (e.g., libmlx4 for the Mellanox ConnectX InfiniBand HCAs). We should either hook into the existing user-space drivers on the system (may be required for proprietary ones) or create Nix expressions for them.