RFE: Specify an existing resource group for the node Resource Group

**What happened**:
[https://docs.microsoft.com/en-us/azure/aks/faq#can-i-provide-my-own-name-for-the-aks-node-resource-group](https://docs.microsoft.com/en-us/azure/aks/faq#can-i-provide-my-own-name-for-the-aks-node-resource-group)
**As you work with the node resource group, keep in mind that you cannot:
• Specify an existing resource group for the node resource group.**
• Specify a different subscription for the node resource group.
• Change the node resource group name after the cluster has been created.
• Specify names for the managed resources within the node resource group.
• Modify or delete Azure-created tags of managed resources within the node resource group. (See additional information in the next section.)

**What you expected to happen**:

Customer has it own processes, creates first its RG and need to specify its name at AKS creation time.

@jnoller @jluk @palma21

**How to reproduce it (as minimally and precisely as possible)**:

**Anything else we need to know?**:

**Environment**:
- Kubernetes version (use `kubectl version`):
- Size of cluster (how many worker nodes are in the cluster?)
- General description of workloads in the cluster (e.g. HTTP microservices, Java app, Ruby on Rails, machine learning, etc.)
- Others: