Optional flag to avoid processing when some condition is met

[RussTorres]

Some pipelines may benefit from checking against an output to avoid re-running pipeline operations. One example of this is in the "pseudo-resumability" built on the asap-pipeline montaging workflow.

For example, with this flag on running generate tilespecs on a collection that already contains all the tileIds that would be generated will result in a success without writing to the render service. There should be an interface to modify this condition for the specific module, but the default for any module should be to run with expected behavior.