This unifies all the builders so that they all wait for SSH using a common
step available in builder/common. This common step has the best practices of
every previously unique step, such as retrying SSH handshakes, limiting
the number of SSH handshakes, timeouts, and so on.

packer: Delete "override" key for config passed to provisioners

The overrides are processed, so that the provisioner only sees the
appropriate sub-settings, so the provisioner does not need the raw
overrides.

If we leave the top-level overrides object in the provisioner
configuration, then it will be rejected as an unknown configuration
key.

[GH-152]

Currently the passed in AWS auth or AWS environment variables are
interpreted by packer. This change moves that logic into goamz in
order to support both the existing and instance based IAM role
authentication. This requires a corresponding change to goamz.