Merge branch 'restrict-tag-keys-values' of https://github.com/lexh/grafana into lexh-restrict-tag-keys-values

Updated sample config and config docs with example for postgres session provider_config string, #2118

When selecting metric tag keys, we only are interested in keys which are
associated with this metric. Likewise, when selecting a value for a
certain key, we only want to consider values which apply to the given
key and metric.

fixing up init script exit codes

Trying to make progres on persisting selection state, restoring selection state for new multi variable dropdown, proving to be really complex

More work on tags in variable dropdown, now the actual tag values query is hooked up and works, #2080