Add config option for package format

This avoids having to type `--pkg-format terraform|cloudformation` every time you want to deploy your code.

Originally requested in https://github.com/aws/chalice/issues/1423.

Start by reading issue 1423 and tracing how the --pkg-format terraform|cloudformation option is parsed and used. Identify the existing configuration-loading entry point, then define what config setting should replace repeated command-line input and verify both package formats still work.

Written by the indexing model from the issue text.