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>

integration-cli/docker_api_containers_test.go

@@ -364,7 +364,7 @@ RUN find /tmp/`,
 	}
 	defer server.Close()
 
-	buf, err := sockRequestRaw("POST", "/build?dockerfile=baz&remote="+server.URL+"/testD", nil, "application/json")
+	buf, err := sockRequestRaw("POST", "/build?dockerfile=baz&remote="+server.URL()+"/testD", nil, "application/json")
 	if err != nil {
 		t.Fatalf("Build failed: %s", err)
 	}