Enforce CLI option dependencies in a user-friendly way

[nchammas]

If you are launching clusters into EC2 (i.e. when --provider is set to ec2), then --ec2-region and --ec2-ami are required.

At the moment we don't enforce this dependency directly in the CLI, partly because Click doesn't directly support option dependencies like this. So if you forget to specify --ec2-ami, it gets caught somewhere in the bowels of Flintrock as opposed to at the front gate, and some ugly error gets barfed out.

This is a user experience problem, and it will get worse when we add support for other providers like GCE since there will be more opportunities for users to forget to specify required options (hah!).

We should fix this.