Add default kernel_name for notebook execution (#497)

Currently, there is no kernel_name. Hence, the execution test cannot run for notebooks that don't have kernels defined in their .ipynb file.

Side-note: We should use lint to remove the kernel_name from .ipynb as well, as it could include info specific to the author's environment.

Co-authored-by: Karl Weinmeister <11586922+kweinmeister@users.noreply.github.com>

.cloud-build/execute_notebook_helper.py

@@ -58,6 +58,7 @@ def execute_notebook(
             output_path=notebook_source,
             progress_bar=should_log_output,
             request_save_on_cell_execute=should_log_output,
+            kernel_name="python3",
             log_output=should_log_output,
             stdout_file=sys.stdout if should_log_output else None,
             stderr_file=sys.stderr if should_log_output else None,