EC2 dynamic inventory: new downcase_facts and normalize_groups options

This adds a new `downcase_facts` option to the ec2.ini configuration for EC2 dynamic inventories (that was discussed on ansible-project mailing list). It defaults to False (the current behavior) so it has to be explicitly enabled. So old configurations without the setting should work just fine. 

It also adds a new `normalize_groups` option that will convert any hyphens into underscores to make it easier to use group names as variables.