Use Unity Catalog for pipelines in the default-python template (#1766)

## Summary

Enables Unity Catalog for pipelines in the default template. Pipelines
will default to non-Unity Catalog pipelines if a catalog is not
specified.

*Small caveat*: there are cases where admins lock down the default
catalog of a workspace and don't allow the creation of a new schema
there. If that happens, the pipeline would fail at runtime with a clear
error indicating what happened. ("PERMISSION_DENIED: User does not have
CREATE SCHEMA on Catalog 'main'."). I've seen this with an internal
Databricks workspace, where creating new non-UC schemas wasn't locked
down, but creation in the `main` was.

## Testing

- Validated on a non-UC + UC workspace. The catalog selection logic here
is the same as applied for the SQL templates.

libs/template/templates/default-python/template/{{.project_name}}/resources/{{.project_name}}_pipeline.yml.tmpl

@@ -3,6 +3,12 @@ resources:
   pipelines:
     {{.project_name}}_pipeline:
       name: {{.project_name}}_pipeline
+      {{- if eq default_catalog ""}}
+      ## Specify the 'catalog' field to configure this pipeline to make use of Unity Catalog:
+      # catalog: catalog_name
+      {{- else}}
+      catalog: {{default_catalog}}
+      {{- end}}
       target: {{.project_name}}_${bundle.environment}
       libraries:
         - notebook: