Move mounting of ebs into instance / docker start script in packer

[benjaminleighton]

Current solution uses a terraform provisioner to mount the ebs after it is associated with the ec2 but this isn't great because timing is a problem vs docker coming up. The docker start logic in instance.sh packer script that runs on ec2 start waits for /certs to exist preventing docker from running before the certs directory is mounted. It isn't a reliable check though because the creation of /certs happens just before it is mounted and theoretically docker could start prior to the mount having completed. A colleague points out that a solution would be to mount the ebs device as part of the docker start logic in instance.sh and avoid the terraform provisioner approach all together.