Initial merge of faas-containerd

This patch completes part of the work in #20 by porting the code
for faas-containerd in-tree. When tested, I was able to deploy
and then remove figlet from the store on `x86_64`.

In a follow-up PR, duplication will be removed where possible
and consolidated with updated documentation.

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

vendor/github.com/openfaas/faas-provider/httputil/writers.go

@@ -0,0 +1,12 @@
+package httputil
+
+import (
+	"fmt"
+	"net/http"
+)
+
+// Errorf sets the response status code and write formats the provided message as the
+// response body
+func Errorf(w http.ResponseWriter, statusCode int, msg string, args ...interface{}) {
+	http.Error(w, fmt.Sprintf(msg, args...), statusCode)
+}