Add GET async=false setting support for grafana server side render possibility

[anryko]

Dashboard server side rendering (for image fetching) will not work at the moment due to Promise usage in the getdash code which is not supported by phantomjs (grafana/grafana#341).

Implement async boolean GET variable to turn of asynchronous behaviour and Promise usage.

Example: http://grafanaIP/render/dashboard-solo/script/getdash.js?host=hostname&metric=cpu&panelId=1&async=false