More defaulting for EC2 inventory plugin.

This branch adds a few defaulting rules for the EC2 inventory plugin:

* If there is no conf file, the defaults from the example `ec2.ini` are used.

* If the `regions` setting is unset, then the environment variables `AWS_REGION` and `AWS_DEFAULT_REGION` are tried (in that order), otherwise, `all` is used.

Please let me know if you'd like me to change anything and resubmit. It's been awhile since I've written much Python and I'm still learning my way around the Ansible code base.