781.environment filtering

Addresses #781 - the minimal form, without pattern matching or an exclusion filter (I think this will do).  The filtering is done in ```config.py``` rather than ```jobfile.py``` so that ```get-config``` shows the filtered environment.  Documented in the suite.rc reference (it should also be mentioned in the new "Suite Design" section awaiting merge in #791 ... after the merge) and tests added.