ec2 inventory script: Add a global group for both EC2 and RDS instances when listing inventory using the EC2 plugin



When using the EC2 inventory plugin as part of an aggregated inventory, it's hard to identify which hosts come from AWS because there is no unified group for all EC2 or RDS instances; they are only tagged based off the attributes that each instance possessed (ie., security group, instance tags, region, etc.)

This change adds 2 global groups, so that all EC2 instances will appear in an 'ec2' group, and all RDS instances in an 'rds' group.

In this way, it's much easier to identify and perform actions to only EC2 or RDS instances because there is now a consistent tag for all hosts of that type.