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!)
- makes packer no more idempotent or janitorial than before
- 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
Do not use latest version of rackspace/gophercloud
Revert "Do not use latest version of rackspace/gophercloud"

This reverts commit c91de565e6e38e46940da081c08e304e262c7b4c.
Do not use latest version of rackspace/gophercloud

- This is not a sustainable dependency management technique
Merge pull request #1622 from delitescere/GH-1533-aws-timeout

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