Right now settings are just passed in through the settings file. This is somewhat limiting if there is a value that will be dynamic with deployments (such as a docker image file, etc.) We should support accepting settings through environment variables, the CLI, the current settings file, and any mix of those.

The order of precedence with mixed should be CLI -> environment -> config file.

If possible, the implementation of this ticket would not be to enumerate each settings value for each of these sources, but to have some way of dynamically finding zappa settings and configuring it.