config: move apply vars to instance config schema

I want to use it outside of the configdata code. It seems appropriate to
expose it from the instance config schema, because it is a general
purpose transformation of instance config data.

Part of https://github.com/tarantool/tarantool-ee/issues/564

NO_DOC=refactoring
NO_CHANGELOG=see NO_DOC
NO_TEST=see NO_DOC