Label selector can be
"set-based"(https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#set-based-requirement)
but such a selector causes Prometheus start failure with the "unexpected
error: parsing YAML file ...: invalid selector: 'foo in (bar,baz)';
can't understand 'baz)'"-like error.

This is caused by the `fields.ParseSelector(string)` function that
simply splits an expression as a CSV-list, so a comma confuses such a
parsing method and lead to the error.

Use `labels.Parse(string)` to use a valid lexer to parse a selector
expression.

Closes #8284.
Signed-off-by: NAlexey Shumkin <Alex.Crezoff@gmail.com>