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

[herbyg-axial]

When using the EC2 inventory plugin in an inventory that is fed by multiple sources (plugins, host file, etc.), it's hard to identify which hosts come from the EC2 inventory because there is no unified grouping that identifies EC2 or RDS instances; they are only tagged based off the attributes that each instance possesses (ie., security group, instance tags, region, etc.)

This change adds 2 global groups:

• all EC2 instances will appear in an 'ec2' group
• 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.

[mpdehaan]

This looks good to me. In queue for testing.

[pas256]

I am curious how many people are using Ansible to manage RDS instances. Perhaps there is a filter that can be added to exclude them.

This is great enhancement though, thanks!

[herbyg-axial]

@pas256 Thanks. I think it's fine to leave the RDS listing in there. It would have been awesome if Ansible allowed you to compose groups in hosts using host patterns, so you could say in your hosts file "[aws:children] all:!rds ", instead of having to filter it out at the plugin level. (Apologies in advance if that's already possible.)

[bcoca]

group_by lets you do this to a certain level, i used to include a
dynamic_groups.yml play at the start of all my plays.

[herbyg-axial]

(Global groups for each type of instance listed by this inventory plugin is useful - can we still get this in?)

[jctanner]

@herbyg-axial thank you! This should be really helpful for those managing heterogeneous environments.