Add feature for invoking namespaced functions

When coupled with the latest version of faas-netes, the gateway
can now invoke, query and deploy functions into alternative
namespaces.

Tested e2e by creating a namespace "fn" and deploying, then
invoking a function deployed there and in the default namespace.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

gateway/handlers/forwarding_proxy_test.go

@@ -135,6 +135,11 @@ func Test_getServiceName(t *testing.T) {
 			url:         "/function/testFunc",
 			serviceName: "testFunc",
 		},
+		{
+			name:        "includes namespace",
+			url:         "/function/test1.fn",
+			serviceName: "test1.fn",
+		},
 		{
 			name:        "can handle request with trailing slash",
 			url:         "/function/testFunc/",