Support for REST get ALL templates.
/_template shows: No handler found for uri [/_template] and method [GET]

It would make sense to list the templates as they are listed in the /_cluster/state call.

Closes #2532.
Remove get index templates deprecated methods
In 0.90.4, we deprecated some code:

* `GetIndexTemplatesRequest#GetIndexTemplatesRequest(String)` moved to `GetIndexTemplatesRequest#GetIndexTemplatesRequest(String...)`
* `GetIndexTemplatesRequest#name(String)` moved to `GetIndexTemplatesRequest#names(String...)`
* `GetIndexTemplatesRequest#name()` moved to `GetIndexTemplatesRequest#names()`

* `GetIndexTemplatesRequestBuilder#GetIndexTemplatesRequestBuilder(IndicesAdminClient, String)` moved to  `GetIndexTemplatesRequestBuilder#GetIndexTemplatesRequestBuilder(IndicesAdminClient, String...)`

* `IndicesAdminClient#prepareGetTemplates(String)` moved to `IndicesAdminClient#prepareGetTemplates(String...)`

* `AbstractIndicesAdminClient#prepareGetTemplates(String)` moved to `AbstractIndicesAdminClient#prepareGetTemplates(String...)`

We can now remove that old methods in 1.0.

**Note**: it breaks the Java API

Relative to #2532.
Closes #3681.