Create an Elasticsearch client for the observer only if needed

[thbkrkr]

Since #2916, we close the ES client at each observation to avoid memory leak, because at each reconciliation we create a new ES client even if we don't need it.

cloud-on-k8s/pkg/controller/elasticsearch/observer/manager.go

Lines 78 to 80 in 85aa75b

	default:
	esClient.Close()
	return observer

It had been identified as a first non-optimal remediation that we had to improve, but ultimately we never did.

#2916 (comment):

I think that instead of passing in a fully instantiated client that gets thrown away most of the time, we should pass a function that creates the client if needed only. This is because creating the client still requires some work like adding the certificates to a trust chain and creating a new HTTP client etc.