Removed the system MPI version of the envrionment, which was
largely unsuccessful.

Attepts to use system MPI were only successful for mpi4py and
not other packages.

More efficient change of permissions.

This is needed so all python versions get built.  Without it,
the python build matrix seems to be ignored.

This is hopefully a temporary hack for just this release.

This allows us to define environments with python dictionaries,
increasing control and flexibility.

The permissions are also more efficient to change with this
approach.

Move env definitions to the top of the script.

Pull out running shell commands into a single function.

Move host-related info higher in the script for easier editing.

E3SM_Diags needs an earlier version of dask, <=2.15.0, to run
correctly with multiprocessing

The script cdscan was not python-3 compatible in cdms2 until a
patch was added in build 11.