Nodespec "nodeSelector" should override Clusterspec

[CodingParsley]

Fixes #284

Description

We have two nodeSelectors at spec level and node level. The expected behavior is that the node's nodeSelector should override the common spec's nodeSelector. So, changing the order.

---

This PR has:

• been tested on a real K8S cluster to ensure creation of a brand new Druid cluster works.
• been tested for backward compatibility on a real K*S cluster by applying the changes introduced here on an existing Druid cluster. If there are any backward incompatible changes then they have been noted in the PR description.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

---

[AdheipSingh]

LGTM thanks @CodingParsley
@xvrl , kindly approve to run workflow + merge.