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.

packer/template.go

@@ -221,6 +221,11 @@ func ParseTemplate(data []byte) (t *Template, err error) {
 			continue
 		}
 
+		// The provisioners not only don't need or want the override settings
+		// (as they are processed as part of the preparation below), but will
+		// actively reject them as invalid configuration.
+		delete(v, "override")
+
 		raw.rawConfig = v
 	}