* 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