Adding same_vpc_only option to EC2 dynamic inventory configuration

##### Issue Type:

Feature Pull Request

##### Ansible Version:

ansible 1.8 (devel ab6e1a43e9) last updated 2014/08/12 15:38:36 (GMT -400)

##### Summary:

This will filter the list of hosts that are returned to be just those in the same VPC where the inventory script is being run

This is the same change as pull request #7916 but in a clean branch without any merge commits in the branch.