Support wildcard Azure subscription VM discovery

[GavinFrazar]

The wildcard "*" can now be used for VM discovery across all subscriptions. The subscriptions discovered will depend on the scope(s) that discovery service identity has Microsoft.Resources/subscriptions/read permission.

TODO in separate PRs:

• Update docs for discovery config and for Azure discovery permissions. Docs reference for static discovery also needs to show the "integration" setting.
• Add support for Azure wildcard subscription join token rule

Changelog: Azure VM discovery configuration now supports specifying a wildcard ("*") subscription to discover all VMs in all subscriptions where the Discovery service has Microsoft.Resources/subscriptions/read permission.

Manual Test Plan

Test Environment

• Create multiple Azure subscriptions
• Create VMs in each subscription
• Create an Azure join token that allows joining from each subscription ID (wildcard Azure subscription join token is not implemented in this PR)
• Create an Azure role for discovery and assign it to the discovery service at a high enough scope to discover each subscription. (I made an assignment for each subscription, but an Azure management group assignment could be used instead)

Test Cases

• Start a discovery service with static config that uses wildcard Azure subscription
  • Verify that all expected VMs are discovered and join the cluster
• Stop discovery service, remove all static configuration except for discovery_group, and disenroll all discovered VMs
• Create a dynamic discovery_config that uses wildcard Azure subscription
  • Verify that all expected VMs are discovered and join the cluster

[GavinFrazar]

to be clear: getAzureSubscriptions is used for setting up kube discovery fetchers. I did not touch it in this PR to keep the scope focused on VM discovery.