Adding subnet-id retrieval from Cluster.

[CorrenSoft]

Fixes #

Description
In scenarios where the VNET is not managed for AKS (meaning it is not present in the node resource group), this script will not correctly populate that value. It is possible to assign that value using environment variables, but that behavior is a little obfuscated and can be confusing at first.

This PR updates the SUBNET_ID logic to also look for the information in the agent profiles, in the following order:

1. Check if the variable already has a value (from Environment).
2. If empty, Check for the value on agentProfiles, using the already loaded AKS_JSON variable.
3. If null, search for the aks-managed subnet in the Nodes Resource Group.

How was this change tested?

• Execute the command and check for the value in karpenter-values.yaml
  chmod +x ./configure-values.sh && ./configure-values.sh ${CLUSTER_NAME} ${RG} karpenter-sa karpentermsi

Does this change impact docs?

• Yes, PR includes docs updates
• Yes, issue opened: #
• No

Release Note

NONE