These shortcut commands make it easy to set enableEtcdTLS and
enableTLSAuth.
`kops set cluster cluster.spec.etcdClusters[*].enableEtcdTLS=true`
`kops set cluster cluster.spec.etcdClusters[*].enableTLSAuth=true`
Introduce an experimental kops set cluster command, for setting
individual fields in the same style as the kops create cluster
--override flags.
For now, feature flag gated by the same SpecOverrideFlag feature flag.
Also split out pkg/commands package to facilitate testing.