ref #9024

If the template variable query has two columns named __text and __value
then return a list of key values. The value is used when the variable is
interpolated in the query. Allows mapping of texts to ids.

Fixes #8855

and a fix for when selecting the same value twice in
a template variable if it is not multi select. It threw an
exception on the second selection

fix: templating issue with data source variables, can now be used right away in other query variables and panels (without page reload), fixes #7510