Validate Kubernetes feature gate input during cluster creation

[fallintoplace]

Summary

• validate Kubernetes feature gate entries before indexing them during kops create cluster
• return errors for empty and sign-only entries instead of panicking or creating an empty feature gate key
• add regression coverage for invalid entries and a valid pass-through case

Root cause

NewCluster indexed featureGate[0] before checking whether the string was empty, and then indexed it again after stripping a leading + or -.

Testing

• go test ./upup/pkg/fi/cloudup -run TestNewClusterValidatesKubernetesFeatureGates
• go test ./cmd/kops -run TestCreateClusterKubernetesFeatureGates

Fixes #18407.

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[ameukam]

/ok-to-test