Allow cluster() to take function/variable values
When referencing different clusters - espcially w/ the advent of Dashboards and Parameters - it's cumbersome to require each cluster reference to use a string literal. These could be better handled and a better experience delivered by allowing the input to
cluster() to take a function or variable value instead of having to be a literal.
This request comes from https://github.com/MicrosoftDocs/dataexplorer-docs/issues/359