Cleans up list workflows

[MattWellie]

Closes #688 (If this is not required, #688 should also be rejected as won't do. Feel free to, I promise I won't cry)

Changes the order of main.py argument processing:

• if list_workflows:

  • List workflows (and end points), then quit

• list_workflows doesn't require the workflow param to be set, but it should be set for all other purposes, so after this return, assert a valid workflow is named

• if list_last_stages:

  • list last stages for the current workflow (this feels redundant with list_workflows already providing this information), then quit

• For all other uses, assert that a valid workflow is defined, and continue as usual

This is a super low priority change, I don't think either of these flags are used by anybody, but maintaining a special case of --workflow list is odd.