We should have a flag to ignore if a resource exists. This is similar to the overwrite: true behaviour but ignoring instead of overwriting.

We could replace the on_exists flag with a different flag for how to respond when a resource exists:

If a resource exists, overwrite:

pipeline:
  name: hello-world
  pipeline: github.com/nextflow-io/hello
  on_exists: overwrite

Which is equivalent to:

pipeline:
  name: hello-world
  pipeline: github.com/nextflow-io/hello
  on_exists: true

If a resource exists, ignore:

pipeline:
  name: hello-world
  pipeline: github.com/nextflow-io/hello
  on_exists: ignore

We could extend this to mean other behaviours through time.