Add support for EC2 2013-08-15 API version - EUCA-7623

A binding is added for the EC2 2013-08-15 API version which includes new
actions:

- DescribeReservedInstancesModifications
- ModifyReservedInstances

with a stub implementation for the describe action.

EUCA-7623
Image registration snapshot ownership test by account - EUCA-7611

Image registration now allows use of a snapshot created by other users
in an account.

EUCA-7611
Merge remote-tracking branch 'origin/dev/sjones/EUCA-7611' into testing
Merge remote-tracking branch 'origin/dev/sjones/EUCA-7623' into testing