Add and use pod labels to avoid having to query all pods from kubernetes

[otrosien]

Expected Behavior

es-operator should only query for pods that belong to EDS resources. We therefore need to attach a custom label to the pods belonging to our EDS, so that we can reduce the memory footprint.

Actual Behavior

This code will make es-operator consume huge amounts of memory in large (2k+ pods) clusters.

Steps to Reproduce the Problem

1. create a huge cluster (doesn't need to be all Elasticsearch)
2. watch es-operator go OOM

Acceptance criteria