Default/Selected (subscription, tenant, user) combination to be used to get an access token

I don't think it is a good idea to try every Azure CLI account, as this may cause **unexpected user credential to be used**.

Instead, only the default/selected **(subscription, tenant, user)** combination should be used to get an access token. If that token doesn't have access, we should explicitly switch account with `az account set`.

_Originally posted by @jiasli in https://github.com/Azure/azure-devops-cli-extension/issues/1226#issuecomment-963889338_