Create a test to cover operator upgrades

[sebgl]

Relates #479.

We should test the operator upgrade, and make sure it does not break existing resources.

A potential scenario:

• spin up a k8s cluster
• apply https://download.elastic.co/downloads/eck/0.9.0/all-in-one.yaml (alpha version)
• apply v1alpha1 Elasticsearch, Kibana & APM resources
• apply https://download.elastic.co/downloads/eck/1.0.0-beta1/all-in-one.yaml (beta version) - should not error out
• make sure the existing alpha cluster is still there untouched
• apply v1beta1 Elasticsearch, Kibana & APM resources
• delete the beta operator statefulset
• apply the current master operator & crd manifests - should not error out
• make sure the existing v1beta1 cluster still works fine
• apply a mutation to the existing v1beta1 cluster, using CRDs v1