[#749] Deprecate UploadConstants.properties, sync property names

* UploadConstant.properties is not needed anymore as the configuration
  data is already in appengine-web.xml
* Use awsAccessKeyId and awsSecretKey property names
Merge pull request #750 from akvo/issue/749_instance_conf_changes

[#749] Deprecate UploadConstants.properties, sync property names