Fix AWS CCM defaults for IPAM to match KCM

[johngmyers]

When reviewing #15623 we noticed that AWS CCM was incorrectly defaulting ClusterCIDR to NonMasqueradeCIDR. Updating the CCM code to match that for KCM.

GCP networking doesn't work on AWS, so removing that code.

I'm not sure whether we should default UseServiceAccountCredentials to true like KCM does.

/cc @hakman

[johngmyers]

KCM also has code for defaulting NodeCIDRMaskSize when ClusterCIDR is IPv6, but we never set ClusterCIDR for IPv6. I don't think we need to copy that.

[johngmyers]

/cc @olemarkus

[hakman]

/cc @justinsb

[justinsb]

Thanks @johngmyers - I also reviewed other uses of the CIDR and think they are correct.

I'm not sure whether we should default UseServiceAccountCredentials to true like KCM does.

It's slightly more secure, in that it allows each controller to have its own serviceaccount, though the process can still assume any of the serviceaccounts IIUC. It feels much more important for KCM, that has a lot of controllers, than it is for a CCM (which has fewer).

/approve
/lgtm

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