Azure VM auto-discovery - specify managed identity client ID

[atburke]

What would you like Teleport to do?
Add an option in the discovery service config to specify the client ID of the managed identity for discovered VMs to use for the Azure join method.

What problem does this solve?
If an Azure VM has more than one user-assigned managed identity, the discovered node will not be able to fetch an access token and will fail to join the cluster.

If a workaround exists, please include it.
Ensure that discovered VMs only have one user-assigned managed identity.