Fixed checking of default build region for EC2

If provider data is in JSON format like this:
"provider":"{\"name\":\"us-east-1\"}"

Then _decode_region_details method properly finds region details, but
a check in snapshot_image_on_provider method fails because it
expects provider in string format only.
Merge pull request #227 from jprovaznik/fixed_region

Fixed checking of default build region for EC2