Elasticsearch version (bin/elasticsearch --version): Affect all versions currently

Description of the problem including expected versus actual behavior:
It is possible to create a new template with a wildcard in the name. My expectation is that this should be prevented, because it leads to difficulty deleting the template individually.

Steps to reproduce:

1. Run: DELETE _template/test-*
2. Run GET _template/test-*
3. Observe there is no template named "test-*"
4. Run:

PUT _template/test-*
{ 
  "order" : 10, 
  "index_patterns" : ["test-*"]
}

5. Run:

PUT _template/test-inner-*
{ 
  "order" : 10, 
  "index_patterns" : ["test-inner-*"]
}

6. Run GET _template/test-*
7. Observe that there is a template named "test-" and "test-inner-"
8. Run DELETE _template/test-*
9. Run GET _template/test-*
10. Observe that both "test-" and "test-inner-" are no longer present

This behaviour is verified on 6.6.2 and 6.7.0 and 7.6.1