Improved scale-up detection.

[ishan16696]

/area high-availability
/kind technical-debt

What this PR does / why we need it:
While testing this PR: gardener/etcd-backup-restore#649 I found out that when third member of etcd cluster won't come up but pod spec has been updated, so etcd statefulset's status .status.updatedReplicas becomes 3:

status:
  availableReplicas: 2
  collisionCount: 0
  currentReplicas: 1
  currentRevision: etcd-main-79f88bd678
  observedGeneration: 2
  readyReplicas: 2
  replicas: 3
  updateRevision: etcd-main-86b48fdf9c
  updatedReplicas: 3

druid remove the scale-up annotation: gardener.cloud/scaled-to-multi-node from etcd statefulset as
func clusterScaledUpToMultiNode(){...} has returned false which is not correct.
more info please refer to this: #587 (comment), now it won't be an issue as backup-restore will skip checking scale-up conditions for first member

Which issue(s) this PR fixes:
Fixes # #587 (comment)

Special notes for your reviewer:

Release note:

None

[shreyas-s-rao]

@ishan16696 thanks for the PR. I feel this may not help solve the corner-case issue we see with scale-up today. Can you please address my comment? Thanks.

[ishan16696]

@unmarshall can you also take a look as I'm addressing your comments in this PR. Thanks

[unmarshall]

/lgtm

[shreyas-s-rao]

MoM of out-of-band call between @ishan16696 ,@unmarshall and myself:

• this PR is not for fixing or mitigating [BUG] Restart of first cluster member can lead to quorum loss while scaling up a non-HA etcd cluster. etcd-backup-restore#646
• it is a result of this discussion
• this PR simply adds an extra layer of protection to prevent wrongful detection of scale-up in a multinode etcd cluster
• we agreed that this code will go away once we introduce deterministic scale-up of the etcd cluster, either via [Feature] Druid-controlled updates to the pods in the etcd cluster #588 or ☂️ [Feature] Add EtcdMember resource #206 (possibly EtcdMember.Spec.InitialClusterState), which needs further discussion

[shreyas-s-rao]

/lgtm