Don't expose overrides directly 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.
Merge pull request #181 from qur/overrides

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