Change pangeo_notebook_version -> image_tag?

[cisaacstern]

My impression is that most contributors will not know or care which tag of the pangeo/forge image we use as a base, so long as it contains their specified version of pangeo_forge_recipes, and any other dependencies they need.

That being said, we will be setting this parameter during recipe parsing an execution, and for certain users (certainly developers) it's a useful option to expose.

So maybe we should deprecate the pangeo_notebook_version field (which as been ignored for some time already) in favor of a more general (optional?) field named something like, image_tag?