Allow eucalyptus region to be configured via ENV

I tried to keep this as simple as possible. 

Eucalyptus users will need to:
* Set "eucalyptus" as the region
* Source their eucarc file or manually export the following environment variables
  * EC2_URL
  * S3_URL
  * EUARE_URL
  * AWS_ELB_URL
  * AWS_AUTOSCALING_URL