remove /v2.1/{tenant_id} from all urls

As discussed at summit, the version part of the URL is not really
relevant, or a thing a user should be filling out themselves, this
should instead be set by the service catalog and extracted from the
token.

This removes it's reference in all documented REST urls, and adds a
new section describing how one gets the base URL for all calls.

Change-Id: I4306b8c3de0225e54f3909dd8a1fb293c4e5944c

api-ref/source/diagnostics.inc

@@ -9,7 +9,7 @@ Shows the usage data for a server.
 Show Server Diagnostics
 =======================
 
-.. rest_method:: GET /v2.1/{tenant_id}/servers/{server_id}/diagnostics
+.. rest_method:: GET /servers/{server_id}/diagnostics
 
 Shows basic usage data for a server.
 
@@ -26,7 +26,7 @@ Request
 
 .. rest_parameters:: parameters.yaml
 
-  - tenant_id: tenant_id
+
   - server_id: server_id
 
 Response