integ-cli: Implement remote FakeStorage server for build via URL tests

Implemented a FakeStorage alternative that supports spinning
up a remote container on DOCKER_TEST_HOST to serve files over
an offline-compiled Go static web server image so that tests which
use URLs in Dockerfile can build them over at the daemon side.

`fakeStorage` function now automatically chooses if it should
use a local httptest.Server or a remote container.

This fixes the following tests when running against a remote
daemon:

- `TestBuildCacheADD`
- `TestBuildCopyWildcardNoFind`
- `TestBuildCopyWildcardCache`
- `TestBuildADDRemoteFileWithCache`
- `TestBuildADDRemoteFileWithoutCache`
- `TestBuildADDRemoteFileMTime`
- `TestBuildADDLocalAndRemoteFilesWithCache`
- `TestBuildADDLocalAndRemoteFilesWithoutCache`
- `TestBuildFromURLWithF`
- `TestBuildApiDockerFileRemote`

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

contrib/httpserver/Dockerfile

@@ -0,0 +1,4 @@
+FROM busybox
+EXPOSE 80/tcp
+COPY httpserver .
+CMD ["./httpserver"]

contrib/httpserver/server.go

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"log"
+	"net/http"
+)
+
+func main() {
+	fs := http.FileServer(http.Dir("/static"))
+	http.Handle("/", fs)
+	log.Panic(http.ListenAndServe(":80", nil))
+}