This change adds support for datasource option to:
* graph panel
* singlestat panel
* templating
This enables the user to use other datasource than the default datasource
for graph/singlestat panels and templating. It is useful when more
than one datasource are available in Grafana.
Change-Id: I2e81a2dc4658c530bf82c4b349e2991dad348b01
Ye Na Rhee