Added better region handling and enabled eu-central-1

Make use of improved connect_to_aws that throws an exception
if a region can't be connected to (e.g. eu-central-1 requires
boto 2.34 onwards)

Add eu-central-1 to the two modules that hardcode their regions
Add us-gov-west-1 to ec2_ami_search to match documentation!
This pull request makes use of the changes in ansible/ansible#9419
Merge pull request #260 from willthames/aws_frankfurt

Added better region handling and enabled eu-central-1, leaving centralizing the list on these modules to a subsequent patch