manifests,pkg/encapsulation: Flannel compatibility

This commit adds basic support to run in compatibility mode with
Flannel. This allows clusters running Flannel as their principal
networking solution to leverage some advances Kilo features. In certain
Flannel setups, the clusters can even leverage muti-cloud. For this, the
cluster needs to either run in a full mesh, or Flannel needs to use the
API server's external IP address.

pkg/mesh/topology_test.go

@@ -979,7 +979,7 @@ func TestRoutes(t *testing.T) {
 			},
 		},
 	} {
-		routes := tc.topology.Routes(kiloIface, privIface, pubIface, tc.local, encapsulation.Never)
+		routes := tc.topology.Routes(kiloIface, privIface, pubIface, tc.local, encapsulation.NewIPIP(encapsulation.Never))
 		if diff := pretty.Compare(routes, tc.result); diff != "" {
 			t.Errorf("test case %q: got diff: %v", tc.name, diff)
 		}