IndexTemplate doesn't support get_template

[selimt]

Currently we can use the IndexTemplate class to save a template as follows:

DocumentClass._index.as_template(template_name).save()

but we can not retrieve an already existing template that uses the es.indices.get_template() api.

We sometimes need this in our unittests to make sure that the template was created correctly.

Can this functionality be added to the IndexTemplate class?