Reduce redundant warning messages in azure-cli credential provider (#410)

## Changes
This warning is a bit noisy: in the happy path, we print it multiple
times, and even if we don't use Azure CLI auth, we may print it anyways.
Also, it will be printed always when using account-level authentication.

This PR reduces the noise for this warning by only printing it when we
are certainly going to use Azure CLI auth, authenticating to a
workspace, and can't get the subscription ID (e.g. because it is not
configured).

## Tests
<!-- 
How is this tested? Please see the checklist below and also describe any
other relevant tests
-->

- [x] Ran locally: with accounts client, no warning is printed, and with
workspace client, only one warning is printed.

Author: mgyucht

databricks/sdk/core.py

@@ -316,7 +316,7 @@ def __init__(self, resource: str, subscription: str = ""):
 
     @staticmethod
     def for_resource(cfg: 'Config', resource: str) -> 'AzureCliTokenSource':
-        subscription = AzureCliTokenSource._get_subscription(cfg)
+        subscription = AzureCliTokenSource.get_subscription(cfg)
         if subscription != "":
             token = AzureCliTokenSource(resource, subscription)
             try:
@@ -327,19 +327,15 @@ def for_resource(cfg: 'Config', resource: str) -> 'AzureCliTokenSource':
                 return token
             except OSError:
                 logger.warning("Failed to get token for subscription. Using resource only token.")
-        else:
-            logger.warning(
-                "azure_workspace_resource_id field not provided. " +
-                "It is recommended to specify this field in the Databricks configuration to avoid authentication errors."
-            )
+
         token = AzureCliTokenSource(resource)
         token.token()
         return token
 
     @staticmethod
-    def _get_subscription(cfg: 'Config') -> str:
+    def get_subscription(cfg: 'Config') -> str:
         resource = cfg.azure_workspace_resource_id
-        if resource == None or resource == "":
+        if resource is None or resource == "":
             return ""
         components = resource.split('/')
         if len(components) < 3:
@@ -368,6 +364,11 @@ def azure_cli(cfg: 'Config') -> Optional[HeaderFactory]:
 
     _ensure_host_present(cfg, lambda resource: AzureCliTokenSource.for_resource(cfg, resource))
     logger.info("Using Azure CLI authentication with AAD tokens")
+    if not cfg.is_account_client and AzureCliTokenSource.get_subscription(cfg) == "":
+        logger.warning(
+            "azure_workspace_resource_id field not provided. "
+            "It is recommended to specify this field in the Databricks configuration to avoid authentication errors."
+        )
 
     def inner() -> Dict[str, str]:
         token = token_source.token()