VPE: Expose runtime image as a pipeline property

[ptitzler]

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

Follow up to #1708, which introduces pipeline properties.

When creating a pipeline, a user must currently define the runtime image for every notebook | script node in the pipeline. For large pipelines this results in a lot of repetitive steps.

Describe the solution you'd like
A clear and concise description of what you want to happen.

• A user can optionally define a default runtime image that will be used if no runtime image is specified in a notebook | script node.
• If a default runtime image is defined in the pipeline properties, the user can override it at the node level.
• The node properties (shown in the node's tool tip and the properties view) should reflect the runtime image that will be used.