updateStrategy support for Beat CRD

[Sayrus]

Proposal

Add support for updateStrategy on Beat CRD. Currently, only the PodTemplate (And replicas on a Deployment) can be modified.

cloud-on-k8s/pkg/controller/beat/common/reconcile.go

Line 116 in e1aa625

PodTemplate: rp.podTemplate,

Use case. Why is this important?
When rolling out a Beat (Filebeat in our case) on large clusters (especially when using DaemonSet), the default updateStrategy's maxUnavailable: 1 increases rollout times. In the case of Filebeat, unless the configuration is invalid, we can actually afford to rollover most of the pods (as long as the burst can be ingested by the ES after rollout).

Note
At first, I thought that daemonSet was mostly passed to the daemonSet itself as the fields description is:

Daemon Set:
  Description:  DaemonSet specifies the Beat should be deployed as a DaemonSet, and allows providing its spec. Cannot be used along with `deployment`. If both are absent a default for the Type is used.

However, as I searched in the code, it does not seem like we are actually allowed to provide any spec for the DaemonSet itself and also had a hard time finding documentation on what fields are allowed inside this daemonSet field. Is there any need for help on this subject?

[Sayrus]

For some reason, I wrote PDB when I meant updateStrategy, sorry about that.