* dashboard: rename var `datasource` to `ds` for consistency reason
Dasbhoards for cluster version or vmagent operate with datasource variable
named `ds`. For consistency sake we rename this variable in single node version
as well.
* dashboard: add instance variable picker
See dashboard reviews here https://grafana.com/grafana/dashboards/10229/reviews
* dashboard: limit number of buckets in histogram to 12 for vmagent dashboard
* dashboard: bump version requirement in description for single version
* dashboard: drop extra series override for single version
* dashboard: set Y-min to zero for most of panels in vmagent dashboard
`vmagent` Grafana dashboard suppose to provide basic observability over multiple
`vmagent` instances. Dashboard is saved in Grafana export format so it can be easily
imported. It was also integrated into docker-compose environment.
