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/os-interface.inc

@@ -12,7 +12,7 @@ and shows details for a port interface.
 List Port Interfaces
 ====================
 
-.. rest_method:: GET /v2.1/{tenant_id}/servers/{server_id}/os-interface
+.. rest_method:: GET /servers/{server_id}/os-interface
 
 Lists port interfaces that are attached to a server.
 
@@ -26,7 +26,7 @@ Request
 
 .. rest_parameters:: parameters.yaml
 
-  - tenant_id: tenant_id
+
   - server_id: server_id_path
 
 Response
@@ -51,7 +51,7 @@ Response
 Create Interface
 ================
 
-.. rest_method:: POST /v2.1/{tenant_id}/servers/{server_id}/os-interface
+.. rest_method:: POST /servers/{server_id}/os-interface
 
 Creates a port interface and uses it to attach a port to a server instance.
 
@@ -65,7 +65,7 @@ Request
 
 .. rest_parameters:: parameters.yaml
 
-  - tenant_id: tenant_id
+
   - server_id: server_id_path
   - interfaceAttachment: interfaceAttachment
   - port_id: port_id
@@ -107,7 +107,7 @@ Response
 Show Port Interface Details
 ===========================
 
-.. rest_method:: GET /v2.1/{tenant_id}/servers/{server_id}/os-interface/{port_id}
+.. rest_method:: GET /servers/{server_id}/os-interface/{port_id}
 
 Shows details for a port interface that is attached to a server.
 
@@ -120,7 +120,7 @@ Request
 
 .. rest_parameters:: parameters.yaml
 
-  - tenant_id: tenant_id
+
   - server_id: server_id_path
   - port_id: port_id_path
 
@@ -146,7 +146,7 @@ Response
 Detach Interface
 ================
 
-.. rest_method:: DELETE /v2.1/{tenant_id}/servers/{server_id}/os-interface/{port_id}
+.. rest_method:: DELETE /servers/{server_id}/os-interface/{port_id}
 
 Detaches a port interface.
 
@@ -160,7 +160,7 @@ Request
 
 .. rest_parameters:: parameters.yaml
 
-  - tenant_id: tenant_id
+
   - server_id: server_id_path
   - port_id: port_id_path