Update mpi4py_available to work around conda-forge/openmpi

[jsiirola]

Fixes # .

Summary/Motivation:

This PR was motivated by conda-forge/openmpi's (inexplicable) implicit hard dependence on cuda in the most recent rebuild of openmpi (and mpich) {see conda-forge/openmpi-feedstock#184). mpi4py was installed and importable in our testing environment, however mpi4py.MPI would raise a linking error because libcudart.so was missing. So, this PR:

• makes sure that mpi4py.MPI is importable when checking if mpi4py is available
• updates various tests to all use pyomo.common.dependencies to check for the availability of mpi4py
• adds a test that will fail if mpi4py is not available and the mpi pytest marker is set (so that we will be able to detect if mpi4py [or its dependencies] cause the MPI tests to be silently skipped again).
• add cudatoolkit cuda-version=11 to the conda environment for MPI tests so that tests resume running. -- [14 Nov 24] conda-forge/openmpi reverted their build, so updating the dependency is no longer needed. The rest of this PR is still useful, as it makes our codebase more robust.

Changes proposed in this PR:

• (see above)

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