[Feature Request]: ADO - Limit duplication of info in vars

### Description

In ADO we have a variable group as well as service connections.

We should discuss if we should recommend use of subscription level svc cons, (giving a default subscription context, still allowing mg level deployments).

If we use information available through the service connection context, we get access to:
- Tenant ID
- Subscription ID
- SPN Client ID & Object ID

Only thing needed as a var is target management group for MG deployments. Default here could also be tenant id, but this being overridden if 'mg_id' is specified as variable.