The name parameter is optional for the CopyImage API call of ec2.

Make the name parameter optional for the copy_image method. The default value is the same as that
of the source AMI being copied [1].

[1]: http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-CopyImage.html
Add unit test for the CopyImage API.

The test case covers two scenarios:
    1. Where all the parameters are passed to the API call.
    2. Where the name parameter is skipped. As per the
    documentation, the name of the source AMI will be used in such a
    case.
Merge remote-tracking branch 'abhinav/fix-copy-image' into develop
Fixes #1672.