Change approach to auto-generating policy

Whilst getting started has been simple, now that I have some custom policies, it's become a pain.

I recommend the following:

On deploy, if .chalice/policy.json exists, use it.
If it doesn't, use .chalice/default-policy.json
If that doesn't, create it using "gen-policy" and/or the auto policy magicks.

This way, it's sill just as easy to get started, and not tedious to deploy once you've decided to write your own policy.json

[I fully admit I may be missing important details about filenames, etc... like why you safe policy.json instead of generating its content when needed if it doesn't exist]