tidy: Context_RtrBGP uses cmd from Cmd_RtrBGP

improves consistency and clarity

asimtote/ios/converters/router/bgp.py

@@ -34,7 +34,7 @@ class Cvt_RtrBGP(Convert):
 
 
 class Context_RtrBGP(Convert):
-    context = "router", "bgp", None
+    context = Cvt_RtrBGP.cmd
 
     def enter(self, asn):
         return ["router bgp " + asn]