Add support for node selector for SG pods

Problem to solve

At this moment, postgres database pods can be randomly placed on any node in the cluster which makes difficult to try to not mix db pods with application pods

Further details

Applications running on the same node than the database, may hurt the performance of the database itself by competing for the same resources that the database needs.

To avoid this problem, most cluster administrators reserves some nodes for databases only.

Proposal

Both under the pods part of the SGCluster.spec. In both cases passing the complete object that K8s supports, without coming up with a new or different syntax, or different set of reduced properties.

• Add support for passing node selectors rules to the pods generated by SG.

Update: refer to comment #184 (comment 375749766) below for further clarification.