ec2_tag: Make it possible to search for EC2 resources by specified tags

On many an occasion, I've wanted the ability to be able to specify from combination of tags and values, and be able to get a list of resource IDs back that match that criteria.  This adds that feature to the **ec2_tag** module.  There is additionally a **resource_type** parameter that can be used to constrain the search down to one particular type of resource for the given tags.