Add a global group for both EC2 and RDS instances when listing inventory from the EC2 inventory 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 of each instance (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 an aggregated inventory where the ec2 plugin is providing a subset of the listed hosts, it's now easy to address all hosts from AWS using one or both of these groups.