Update rkt API version to 1.6.0

Godeps/Godeps.json

@@ -545,8 +545,8 @@
 		},
 		{
 			"ImportPath": "github.com/coreos/rkt/api/v1alpha",
-			"Comment": "v1.4.0",
-			"Rev": "f5b69782d120a7ae5ada8f2ebad97a014a98ebd5"
+			"Comment": "v1.6.0",
+			"Rev": "14437382a98e5ebeb6cafb57cff445370e3f7d56"
 		},
 		{
 			"ImportPath": "github.com/cpuguy83/go-md2man/md2man",

vendor/github.com/coreos/rkt/api/v1alpha/api.proto

@@ -217,6 +217,10 @@ message PodFilter {
 
         // If not empty, the pods whose cgroup are listed will be returned.
         repeated string cgroups = 7;
+
+        // If not empty, the pods whose these cgroup belong to will be returned.
+        // i.e. the pod's cgroup is a prefix of the specified cgroup
+        repeated string pod_sub_cgroups = 8;
 }
 
 // ImageFilter defines the condition that the returned images need to satisfy in ListImages().