string constants in the API should match Kubernetes style

[errordeveloper]

Part of #888.

We currently have some constants in Kubernetes style, but some aren't.

Namely, we have vpc.nat.gateway where the constants are Disable, Single and HighlyAvailable.

But in nodegroup.ami we use have lower-case.

We also have constants that are defined by AWS, e.g. in cloudWatch.clusterLogging.enableTypes.

[Himangini]

We need to wait for eksctl v1 before proceeding with this change.