Add an option to ec2.ini to filter instances based on various things

(This is try two for the PR, replacing #7359 which ended up with merge commits. See the previous PR for a bit of commentary.)

This lets you filter instances right at the EC2 API level using the EC2 inventory plugin. You can use this to retrieve only instances with certain tags, or only certain instance types, etc, etc. See ec2.ini for more info and examples.