Fix thanos sidecar selector

Previous PR[1] which deprecated `thanosSelector` accidentally broke the
usage of it. This commit restores the old behaviour along with newly
introduced `thanos` config variable.

[1] https://github.com/prometheus-operator/kube-prometheus/pull/1543



Co-authored-by: Pawe? Krupa <pawel@krupa.net.pl>
Signed-off-by: Arunprasad Rajkumar <arajkuma@redhat.com>