Update validation for master instance groups

[justinsb]

We need update validation for updates to the instance groups, in particular the master instance groups. The following should probably not be allowed:

• Changing the role
• Changing the count of instances (?)
• Changing the zones for the IG

(While we're here, we might as well also validate that the cluster etcd clusters aren't resized)

[krisnova]

While we're here, we might as well also validate that the cluster etcd clusters aren't resized

I feel like there is another issue floating around for this very thing.. am I wrong?

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[chrislovecnm]

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This is a good starter issue

1. test to see if you can do these things
2. add validation to one of the appropriate files in here: https://github.com/kubernetes/kops/tree/master/pkg/apis/kops/validation
3. do not add the validation to legacy.go