[EC2]Correct the return status of attaching volume

The EC2 API return volume's status as 'detached' in response when
attaching a volume.
It should be 'attaching' according to EC2's API docs, so we should convert
the volume_status or volume attach_status to ec2 volume attachment status.
The situation is similar in detaching volume.

Change-Id: Ifacf0ffcab49394def948d428c4c4c13d08152f0
Closes-bug: #1280572
Merge "[EC2]Correct the return status of attaching volume"