Set a dynamic MinAvailable value for the default PDB

[sebgl]

Fixes #916.

Reconcile the default PDB to have its MinAvailable value consider the current number of Pods we expect to run, according to the StatefulSet specs.

This is done right after handling nodes upscale: we work with the returned (updated) StatefulSets. Summing their replicas give us the maximum number of nodes we should have running. Some of them may not be running yet, but that's OK since it will lead to a higher MinAvailable value (-> no disruption allowed).

By default, we want to set MinAvailable to NumberOfPods - 1, so only one Pod can be disrupted.
However, in cases such as:

• cluster not green (best-effort based on out-of-date information)
• only 1 master node
• only 1 data node

we don't want any Pod to be disrupted.

Side-effects of this PR in the code:

• return the updated StatefulSets at the end of the upscale phase
• add a typed ElasticsearchUnknownHealth const

Follow-up issues:

• Update PDB instead of delete + create on k8s > 1.15 #1773 to Update() instead of Delete()+Create() on k8s 1.15+
• Improve PDB cluster checks #1774 to improve the cluster-wide checks

[david-kow]

Looks good overall, just few comments.

[thbkrkr]

Typo/wording: If one already exists with the same namespace & name

[barkbay]

LGTM

[david-kow]

LGTM