Generalize config

I've made some changes in order to be able to use amzn-base in our environment.

I've set packer up to default to AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY and AWS_DEFAULT_REGION environment variables (though they can of course still be set as `-var` arguments). This mirrors what the `aws cli` tool will use for configuration if not specified explicitly via a config file or on the command line.

I've also changed `build-ami` so that it requires a `source_ami` as a second argument and set `instance_type` up as a variable defaulting to `m1.large`.

I know amzn-base is opinionated software, so I hope these changes are useful but my feelings won't be hurt if you don't want to pull in all of the changes :).