refs #8886 this should fix it

As no check if key is missing was implemented
refs #8886 Unit test modified to check failing case as well.
refs #8886 fixing unit test
refs #8886 Fixing unit test/DgreduceTest.py

(previous one went to wrong place)
Merge remote-tracking branch 'origin/bugfix/8886_dgreduceHardmaskfileIssue'