Add server URL option #47
Description
By default, the viewer config of a compute engine (i.e. image generated by xcetool) assumes that it will find the viewer API at http://localhost:8080. This can be overridden directly with the --xcube-viewer-api-url parameter at runtime, or indirectly (when running on localhost) with xcetool image run --port. But if the deployment URL is already known at the compute engine's build time, it would be useful to be able to specify this as a default via the xcengine_config dictionary in the notebook.