auto-scaling: Correct caveats

The currently formula caveats uses the [AmazonWebServicesFormula caveats](https://github.com/Homebrew/homebrew/blob/c5ea5dfe23d5c96a34c331261fbab50feb641d13/Library/Homebrew/formula_specialties.rb#L30-L44) which recommend adding AWS_ACCESS_KEY and AWS_SECRET_KEY.

These will not work. You must provide a credential file as per the instructions in the auto-scaling cli instllation guide:

http://docs.aws.amazon.com/AutoScaling/latest/DeveloperGuide/UsingTheCommandLineTools.html

> After you sign up for AWS, you must create access keys for the account. Your access keys consists of an Access Key ID and a Secret Access Key. You must provide your access keys to the command line tool so that they know that the commands that you issue come from your account. <strong>The command line tool reads your access keys from a credential file that you create on your local system.</strong>
>
> You can either specify your credential file with the --aws-credential-file parameter every time you issue a command, or you can create an environment variable that points to the credential file on your local system. If the environment variable is properly configured, you can omit the --aws-credential-file parameter when you issue a command. The following procedure describes how to create a credential file and a corresponding AWS_CREDENTIAL_FILE environment variable.

Note that the EC2 command line tools still appear to use the other environment variables:

http://docs.aws.amazon.com/AWSEC2/latest/CommandLineReference/set-up-ec2-cli-linux.html

> The easiest way to provide your access keys to the Amazon EC2 CLI is to set the AWS_ACCESS_KEY and AWS_SECRET_KEY environment variables.
Or do you mean replace all of the scripts the package installs with shim scripts which sets the variables it can and complains about the others? The scripts themselves do this somewhat.