Add MPI environment variables to packages

[adamjstewart]

This PR solves #991, thanks to the brilliant @alalazo! parallel-netcdf wouldn't build for me with Intel and OpenMPI without these environment variables set. It's probably a good idea to have them in all MPI packages.

[alalazo]

@adamjstewart Don't forget to change :

self.spec['mpi'].mpicc # and similar

into

mpicc # an similar

in every package that used the feature. scotch could be a good test case to check that the changes do not actually break compilation.

[davydden]

@adamjstewart documentation should also be adjusted accordingly.

[adamjstewart]

Oh, is that what that does? I kinda like spec['mpi'].mpicc, it felt less magical and more understandable. I still don't know what spack_cc means, but I see that around various places.

I think I'll back out the second commit since that affects a lot of packages and isn't really the same issue that I'm facing. If someone else wants to create a second PR to set module.mpicc and edit every package and the documentation then they're welcome to do so.

[davydden]

I kinda like spec['mpi'].mpicc, it felt less magical and more understandable.

+1.