Updated tag behavior for AutoScale Groups

This pull request is intended to make the ec2_asg behave similar to the ec2 module with regards to tagging:

Currently the module expects tags in this form:

    tags:
      - key: environment
        value: production
        propagate_at_launch: no


With this patch, it can be streamlined to:

    tags:
      - environment: production
        propagate_at_launch: no

And makes it in line with the ec2 module's behavior:

    instance_tags:
        db: postgres