This adds retry logic to the amazon/chroot builder.  The builder
intermittently fails when ec2conn shows the volume in the attached
state but calling Volumes[0].Attachments return an empty array. The
fix adds a retry logic for when Volumes[0].Attachments has len 0 sleep
for 2 seconds and retry up to 30 times.

When the Volumes[0].Attachments is empty I find within 5 seconds the
Volumes[0].Attachments contains the correct value.

The issue is reported in:
https://github.com/mitchellh/packer/issues/1854