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]