Add "router add port" to osc

This patch proposes the migration of port addition of
Neutron's "router-interface-add" to OSC's "router add port".

Change-Id: I3a8a76d384caa32975f8e77f50f3cf3c3be13786
Closes-Bug: #1546849
Depends-On: I9783bc4ccceae3d361dce52d51483ef2187920a9
Implements: blueprint neutron-client-advanced-router

doc/source/command-objects/router.rst

@@ -4,6 +4,27 @@ router
 
 Network v2
 
+router add port
+---------------
+Add a port to a router
+
+.. program:: router add port
+.. code:: bash
+
+    os router add port
+        <router>
+        <port>
+
+.. _router_add_port:
+
+.. describe:: <router>
+
+    Router to which port will be added (name or ID)
+
+.. describe:: <port>
+
+    Port to be added (name or ID)
+
 router create
 -------------
 
@@ -16,7 +37,7 @@ Create new router
         [--project <project> [--project-domain <project-domain>]]
         [--enable | --disable]
         [--distributed]
-	[--availability-zone-hint <availability-zone>]
+        [--availability-zone-hint <availability-zone>]
         <name>
 
 .. option:: --project <project>
@@ -146,4 +167,4 @@ Display router details
 .. _router_show-router:
 .. describe:: <router>
 
-    Router to display (name or ID)
+    Router to display (name or ID)