helm chart: Support K8S 1.8+

[mumoshu]

I have encountered docker-library/elasticsearch#111 after installing a cluster with helm.

Digging a bit, it turned out that K8S 1.8+ dropped support for annotation-based init containers so that the init container wasn't run at all in my env(K8S 1.9).

Rewriting the chart template to use an initContainers field rather than an annotation would fix the issue.

[mumoshu]

This and #138 seem to share the same cause.

[mumoshu]

Let me share what worked for me:

initContainers:
      - name: sysctl
        image: busybox
        imagePullPolicy: IfNotPresent
        command: ["sysctl", "-w", "vm.max_map_count=262144"]
        securityContext: { "privileged": true }
containers:
# snip

[mumoshu]

Turns out you'd better use the startup-script-daemonset approach than initContainers because the latter doesn't survive node restarts. See pires/kubernetes-elasticsearch-cluster#85

[stevesloka]

I'm working on an update now to add the daemonset approach.

[stevesloka]

Fixes in #149