Changes step success to mean successful execution (e.g. excluding skipped steps)
Makes sure pipeline success is not affected by skipped steps in the process.
Adds a pipeline execution test with optional output solids.
Tracked in https://github.com/dagster-io/dagster/issues/1380
Test Plan: unit
Reviewers: #ft, alangenfeld
Reviewed By: #ft, alangenfeld
Differential Revision: https://dagster.phacility.com/D775