LU-3889 tests: fix parallel-scale typo and allow overrides

LU-3889 tests: fix parallel-scale typo and allow overrides

This fixes a typo introduced by change #7778.  This also allows
overriding all variables that are set lower due to SLOW=no, which is
forced for ZFS tests.  This will allow values specified by caller for
all variables even when testing on ZFS.

Test-Parameters: envdefinitions=SLOW=yes mdtfilesystemtype=zfs mdsfilesystemtype=zfs ostfilesystemtype=zfs testlist=parallel-scale
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I5b585ffa28c9f7d729d5169ff96c383cc0af38d3
Reviewed-on: http://review.whamcloud.com/8234
Reviewed-by: Cliff White <cliff.white@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>