[helm loki/promtail] make UpdateStrategy configurable

[stefanandres]

This commit makes UpdateStrategy of the promtail daemonset configurable.
On large installations, you may want to increase the value of maxUnavailable.

This fixes #1878

What this PR does / why we need it:

See #1897

Which issue(s) this PR fixes:
Fixes #1897

Special notes for your reviewer:

I've reused the existing variable. This may break when you have this configured to the non-default value. Please advice if I shall add another variable or something else.

The codeis inspired by https://github.com/helm/charts/blob/master/stable/nginx-ingress/values.yaml#L139 and https://github.com/helm/charts/blob/master/stable/nginx-ingress/templates/controller-daemonset.yaml#L25

Checklist

• Documentation added
• Tests updated

[stefanandres]

The new default will change:

    updateStrategy:
-     rollingUpdate:
-       maxUnavailable: 1
-     type: RollingUpdate
+     {}

This will also now default to the k8s default, which is RollingUpdate with maxUnavailable: 1 (like before)

$ kubectl get daemonset -o yaml|grep -i -A3 updatestrategy:
    updateStrategy:
      rollingUpdate:
        maxUnavailable: 1
      type: RollingUpdate
--
    updateStrategy:
      rollingUpdate:
        maxUnavailable: 1
      type: RollingUpdate

[cyriltovena]

LGTM