Sometimes you may want to allow variables through host_vars or inventory, but prompt for a value if it is not set or if the value does not conform to something specific. This option allows you to specify when you want to offer a prompt.

This patch also moves check_conditional to utils, and adds an is_unset() function which is nicer to read:

    only_if: "not is_set('${var}')"

vs

    only_if: "is_unset('${var}')"