Add EtcdCluster resource examples

[kvaps]

following up #109 (comment)

Here are examples for:

• Simple cluster with few replicas
• Cluster with custom image
• Cluster with persistent storage
• Cluster with resources
• Cluster with all field specified

[kvaps]

I’ll personally raise the question again whether we should replace:

spec.pod --> spec.podTemplate
spec.service --> spec.serviceTemplate
spec.podDisruptionBudget --> spec.podDisruptionBudgetTemplate

piraeus-operator and elk-operator use it this way

[sircthulhu]

lgtm

I would personally add etcdcluster-with-default-pdb.yaml

---
apiVersion: etcd.aenix.io/v1alpha1
kind: EtcdCluster
metadata:
  name: test
  namespace: default
spec:
  replicas: 3
  podDisruptionBudget: {}

[kvaps]

PR rebased after internal discussion to use Template suffixes for configuring Kubernetes entities.

Another advantage of using the Template suffixes is that it serves as a contract to adhere to the Kubernetes specifications.

What we have in storage and options is validated and processed by our logic, and the specifications there are strictly our own. However, what is in *Template follows the Kubernetes specifications and is intuitively understandable.