Add workflow linter

[TomAugspurger]

Is your feature request related to a problem? Please describe.

We should have a linter that validates workflows files, checking for common mistakes prior to submission.

• verify task is an object like mypackage.mymodule:MyClass. Naive users (cough me cough) might accidentally put mypackage.mymodule.MyClass (using a . instead of a :)
• verify that the image has certain runtime properties (e.g. pctasks.* installed). https://github.com/NVIDIA/container-canary might be a nice tool to use here.

(feel free to add more requests here)