Adding regions_exclude parameter to be able to do subtractive region exc...

...lusion. 

Rationale:  boto 2.11.0 gives access to GovCloud, but GovCloud requires GovCloud credentials for api access.  This was breaking ec2.py because us-gov-west-1 now gets returned by ec2.regions(), so when ec2.py tried to scan that region, it would return a 401 Unavailable.

I set the default to us-gov-west-1 since that probably makes sense for most users, lest all of their things break when they upgrade boto.