builder/amazon: Extend timeout and allow user override [GH-1533]

- help resolve https://github.com/mitchellh/packer/issues/1533
  (although timeouts are always ultimately useless in a distributed
  system!)
- derive maximum number of ticks from timeout
- default timeout to 300s (5m) to cater for global AMI copying
- allow user to override with AWS_TIMEOUT_SECONDS environment variable