Changes required to facilitate running of pp_demo_suite using head of dagrunner

[cpelley]

Reverted change from #40 involving common args handling and filled in gap in testing to ensure we continue to support reliant behaviour.

Working pp_demo_workflow when using this dagrunner dev branch:
http://fcm1/cylc-review/taskjobs/cpelley?&suite=demo_workflow_dagrunner_head&no_fuzzy_time=0&tasks=execute_basic_model

Best explained from looking at the new testcase:

dagrunner/dagrunner/tests/execute_graph/test_plugin_executor.py

Lines 71 to 73 in 8278841

	Passing 'common args' to a plugin that doesn't have such arguments
	defined in its signature. Instead, filter out those that aren't
	specified in the graph.

Issues

• https://github.com/MetOffice/improver_suite/issues/2253