elasticsearch_configure provider should not modify default resource parameters

[martinb3]

The elasticsearch_configure provider is currently writing to new_resource.default_configuration:

    if new_resource.path_conf[es_install.type] && new_resource.default_configuration['path.conf'].nil?
      new_resource.default_configuration['path.conf'] = new_resource.path_conf[es_install.type]

This isn't a problem when only creating a single resource, but if subsequent resources of the same type are defined, they are receiving the modified new_resource.default_configuration value, and then have a bunch of unexpected settings from the previous resource.

This is causing issues for people who run more than one instance, e.g. #444.