process_robot_output_files: Default gzip arg to env var GZIP
process_robot_output_files: Default gzip arg to env var GZIP

Also, misc. style improvements

Change-Id: I4e9d757a71dd78b122e03a091e58b32c32375702
Signed-off-by: Michael Walsh <10404802c7de463f402c4a18638106ca1b12ef61@us.ibm.com>

openbmc/openbmc-test-automation,openbmc/openbmc-test-automation