Propose a post-submit matrix

Propose a post-submit matrix

A recent bug would have been prevented if some testing of
separate PME ranks had been occuring.

Extended gromacs.py to permit regressiontests to run
with separate PME ranks, and/or designated numbers of
ranks, or GPU ids.

Quieted some newly exposed warnings.

Documented more of the reasoning behind matrix choices, to help us
maintain better. Noted various TODOs for better testing coverage.

Refs #2134

Change-Id: Ib9828ca769d7a446c61fb8bb7a68128a38991aba