Add support for custom annotations

[komljen]

I needed this feature for kube2iam, hope it is useful for others as well.
Example:

apiVersion: enterprises.upmc.com/v1
kind: ElasticsearchCluster
metadata:
  name: example-es-cluster
spec:
  elastic-search-image: upmcenterprises/docker-elasticsearch-kubernetes:6.1.3_0
  client-node-replicas: 1
  master-node-replicas: 1
  data-node-replicas: 1
  network-host: 0.0.0.0
  annotations:
    iam.amazonaws.com/role: k8s-test-es
  zones: []
  data-volume-size: 15Gi
  java-options: "-Xms512m -Xmx512m"
  snapshot:
    scheduler-enabled: false
  storage:
    type: gp2
    storage-class-provisioner: kubernetes.io/aws-ebs
  resources:
    requests:
      memory: 512Mi
      cpu: 500m
    limits:
      memory: 1024Mi
      cpu: '1'

The annotations are applied to master, data and client pods.

[komljen]

@stevesloka I already tested this PR, and also this one #308. Both PRs are merged in this image komljen/elasticsearch-operator:0.3.0-2 if someone wants to test it.

[stevesloka]

@komljen could you rebase?

[komljen]

@stevesloka done!