update subnet tags and validator script

[oliviassss]

Issue #, if available:

Description of changes:

1. Update the /12 subnets tags to include the tag for vpc-cni subnet discovery.
2. Update the validator script to fix an issue the aws cli to list cidr reservations in subnets

Manually verified the scripts

% ./tests/assets/eks_cluster_validator.sh                                                                                              
Checking aws-node VPC CNI version...
[PASS] aws-node VPC CNI version 1.19.5 is >= 1.19.4
Checking aws-node container environment variables...
[PASS] DISABLE_LEAKED_ENI_CLEANUP is set to 'true'
[PASS] ENABLE_PREFIX_DELEGATION is set to 'true'
[PASS] MINIMUM_IP_TARGET is set to '30'
[PASS] WARM_IP_TARGET is set to '5'
Checking VPC subnets for /12 CIDR blocks and prefix delegation...
VPC_ID: vpc-01ac18664562057a3
[WARN] Subnet subnet-aaa has /16 CIDR block (not /12)
[WARN] Subnet subnet-bbb has /16 CIDR block (not /12)
[PASS] Subnet subnet-ccc has /12 CIDR block and prefix reservations
[PASS] Subnet subnet-ddd has /12 CIDR block and prefix reservations
...
[PASS] Found 6 subnets with /12 CIDR blocks and prefix delegation
All validations PASSED.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[hakuna-matatah]

/lgtm