Add `--wait` to commands where this flag is missing

[nikimanoledaki]

Description

Following up from #5250, the aim of this ticket is to find commands that would benefit from --wait but are missing this flag, and add the flag to these commands.

For example, some suggestions:

• eksctl drain nodegroup
• eksctl scale nodegroup
• eksctl deregister/register cluster

[github-actions[bot]]

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[Himangini]

As part of this ticket we'll add --wait flag for the following command

• eksctl drain nodegroup

We'll open a new ticket for the following commands

• eksctl register cluster
• eksctl deregister cluster

[TiberiuGC]

After further investigation it turns out that eksctl scale nodegroup is the single command out of the ones mentioned in this ticket that can really benefit from supporting the --wait flag. As for the rest:

• eksctl drain nodegroup - this command already waits by default for pods to be evicted from the nodes https://github.com/weaveworks/eksctl/blob/f261471677c75703d0da0d643eb3c1ed268ca3d4/pkg/drain/nodegroup.go#L205-L247
• eksctl register / deregister cluster - the EKS API calls used for these commands don't trigger any async process. So there's nothing to wait on.