Ansible aws_ec2 plugin how to set credentials for discovered hosts?

Need to launch ad-hook commands like "-m ping" on existing ec2 instances, but it requred key-pair.

How to set key-pair for boto, like "aws_access_key_id" stored in ~/.aws/credentials ?

Also, have a problem invertory:i got "invertory" folder near Ansible, where stored both local hosts and aws_ec2.yml file. But ansible-invertory --list works only for aws_ec2.yml file...