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_test.go

@@ -20,6 +20,7 @@ func TestLoadingSettings(t *testing.T) {
 			So(err, ShouldBeNil)
 
 			So(AdminUser, ShouldEqual, "admin")
+			So(cfg.RendererCallbackUrl, ShouldEqual, "http://localhost:3000/")
 		})
 
 		Convey("Should be able to override via environment variables", func() {
@@ -178,5 +179,15 @@ func TestLoadingSettings(t *testing.T) {
 			So(InstanceName, ShouldEqual, hostname)
 		})
 
+		Convey("Reading callback_url should add trailing slash", func() {
+			cfg := NewCfg()
+			cfg.Load(&CommandLineArgs{
+				HomePath: "../../",
+				Args:     []string{"cfg:rendering.callback_url=http://myserver/renderer"},
+			})
+
+			So(cfg.RendererCallbackUrl, ShouldEqual, "http://myserver/renderer/")
+		})
+
 	})
 }