bootstrap uses incorrect IP when multiple ENI exists on node

[M00nF1sh]

What happened:
when multiple ENI exists on node prior to bootstrap, the IP address of ENI other than eth0 may be used and passed to kubelet.
Which then may fail kubelet's IP address validation

What you expected to happen:
we should always use eth0's IP address when generate node's IP.

How to reproduce it (as minimally and precisely as possible):
keep launch nodes with 2 enis, and sometimes node won't get IP.

Anything else we need to know?:
In https://github.com/awslabs/amazon-eks-ami/blob/master/files/bootstrap.sh#L430 we used the first MAC instead of eth0's mac.
i believe we should just use latest/meta-data/mac (The instance's media access control (MAC) address. In cases where multiple network interfaces are present, this refers to the eth0 device (the device for which the device number is 0).)

The code change was introduced in #773

Environment:

• AWS Region: us-west-2
• Instance Type(s): c5.9xlarge
• EKS Platform version (use aws eks describe-cluster --name <name> --query cluster.platformVersion):
• Kubernetes version (use aws eks describe-cluster --name <name> --query cluster.version):
• AMI Version:
• Kernel (e.g. uname -a):
• Release information (run cat /etc/eks/release on a node):

[cartermckinnon]

Proposed change makes sense to me. This would only effect ipv6, right? Because the MAC is only used in the ipv6 cases.