improve remote image rendering (#13102)

* improve remote image rendering

- determine "domain" during Init() so we are not re-parsing settings
  on every request
- if using http-mode via a rednererUrl, then use the AppUrl for the
  page that the renderer loads.  When in http-mode the renderer is likely
  running on another server so trying to use the localhost or even the
  specific IP:PORT grafana is listening on wont work.
- apply the request timeout via a context rather then directly on the http client.
- use a global http client so we can take advantage of connection re-use
- log and handle errors better.

* ensure imagesDir exists

* allow users to define callback_url for remote rendering

- allow users to define the url that a remote rendering service
  should use for connecting back to the grafana instance.
  By default the "root_url" is used.

* improve remote image rendering

- determine "domain" during Init() so we are not re-parsing settings
  on every request
- if using http-mode via a rednererUrl, then use the AppUrl for the
  page that the renderer loads.  When in http-mode the renderer is likely
  running on another server so trying to use the localhost or even the
  specific IP:PORT grafana is listening on wont work.
- apply the request timeout via a context rather then directly on the http client.
- use a global http client so we can take advantage of connection re-use
- log and handle errors better.

* ensure imagesDir exists

* allow users to define callback_url for remote rendering

- allow users to define the url that a remote rendering service
  should use for connecting back to the grafana instance.
  By default the "root_url" is used.

* rendering: fixed issue with renderKey where userId and orgId was in mixed up, added test for RenderCallbackUrl reading logic

pkg/setting/setting.go

@@ -197,6 +197,7 @@ type Cfg struct {
 	ImagesDir                        string
 	PhantomDir                       string
 	RendererUrl                      string
+	RendererCallbackUrl              string
 	DisableBruteForceLoginProtection bool
 
 	TempDataLifetime time.Duration
@@ -641,6 +642,18 @@ func (cfg *Cfg) Load(args *CommandLineArgs) error {
 	// Rendering
 	renderSec := iniFile.Section("rendering")
 	cfg.RendererUrl = renderSec.Key("server_url").String()
+	cfg.RendererCallbackUrl = renderSec.Key("callback_url").String()
+	if cfg.RendererCallbackUrl == "" {
+		cfg.RendererCallbackUrl = AppUrl
+	} else {
+		if cfg.RendererCallbackUrl[len(cfg.RendererCallbackUrl)-1] != '/' {
+			cfg.RendererCallbackUrl += "/"
+		}
+		_, err := url.Parse(cfg.RendererCallbackUrl)
+		if err != nil {
+			log.Fatal(4, "Invalid callback_url(%s): %s", cfg.RendererCallbackUrl, err)
+		}
+	}
 	cfg.ImagesDir = filepath.Join(DataPath, "png")
 	cfg.PhantomDir = filepath.Join(HomePath, "tools/phantomjs")
 	cfg.TempDataLifetime = iniFile.Section("paths").Key("temp_data_lifetime").MustDuration(time.Second * 3600 * 24)