Rephrase rightscale description in docs
Make provider_extras logic work for ServerDeployer.

This change allows passing provider-specific server launch arguments.
E.g. The following server config snippet assigns an IAM role to a server:

    servers:
      minimal:
        # other attributes omitted for brevity
        provider: aws
        aws:
          instance_profile_name: instance_profile_for_iam_role
Add example of a minimal server in AWS (including IAM role assignment).