add c7g support

[lizthegrey]

Description of changes:
See aws/amazon-vpc-cni-k8s#1940

Without this change, bootstrap fails:

+ /etc/eks/bootstrap.sh dogfood-eks --kubelet-extra-args --node-labels=eks.amazo
naws.com/capacityType=ON_DEMAND,eks.amazonaws.com/nodegroup=custom-shared-0
No entry for type 'c7g.2xlarge' in /etc/eks/eni-max-pods.txt. Will attempt to au
to-discover value.

An error occurred (InvalidInstanceType) when calling the DescribeInstanceTypes o
peration: The following supplied instance types do not exist: [c7g.2xlarge]
Exited with error on line 437
Apr 26 15:42:19 cloud-init[1269]: util.py[WARNING]: Failed running /var/lib/clou
d/instance/scripts/part-001 [255]
Apr 26 15:42:19 cloud-init[1269]: cc_scripts_user.py[WARNING]: Failed to run mod
ule scripts-user (scripts in /var/lib/cloud/instance/scripts)
Apr 26 15:42:19 cloud-init[1269]: util.py[WARNING]: Running module scripts-user 
(<module 'cloudinit.config.cc_scripts_user' from '/usr/lib/python2.7/site-packag
es/cloudinit/config/cc_scripts_user.pyc'>) failed
Cloud-init v. 19.3-45.amzn2 finished at Tue, 26 Apr 2022 15:42:19 +0000. Datasou
rce DataSourceEc2.  Up 70.30 seconds

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[suket22]

Thanks for making the change!

[lizthegrey]

Fix confirmed, new AMI works like a charm.