Align volume delete on terminate with AWS defaults

Updated the BlockDeviceType to have a default to terminate volumes on
instance termination, the same as the AWS default as documented in these
pages:
  http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/block-device-mapping-concepts.html
  http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-RegisterImage.html

Also updated register_image to allow for users to explicitly set their
delete on terminate preference when creating an image from a snapshot.

Updated tests to work with the new default, and created new tests to
assert that the default behavior is in place.
Reverting changes to default delete on terminate

This commit leaves the new option present on register_image, but keeps
the default of not deleting volumes on instance termination.
Merge branch 'delete-on-terminate-optional' of https://github.com/adamsb6/boto into develop

Update EC2 `register_image` to expose an option which sets whether an instance
store is deleted on termination. The default value is left alone.