ec2/11-attach-volume: Wait for volume to be available

Prevents this from happening:
    Run task: 10-create-volume
    The EBS volume id is vol-deadbeef
    Run task: 11-attach-volume
    IncorrectState: vol-deadbeef is not 'available'.
Merge pull request #85 from soult/master

ec2/11-attach-volume: Wait for volume to be available