added mapnode argument to allow enforce serial execution
Added mapnode single input test
Updated deprecated  "workflow.config['crashdump_dir']" to
"workflow.config['execution']['crashdump_dir']" in other tests in
test_engine.py
line adjusted
Added mapnode "serial" input argument comment description
Minor streamlining of serial argument test
Merge remote-tracking branch 'origin/pr/851'
serial mapnode