Use a class provided logger before falling back to module

If a subclass overrides the logging listeners and provides
a class specific logger (under the attribute named _LOGGER)
try to use that before using the taskflow specific logging
module (and only use this one as a last resort).

This allows subclasses to easily override the default logger
without having to continually pass in a constructor argument
to do the same.

Change-Id: I91fea3db6cdd1dfb39963ff9589fd530fe087278

taskflow/utils/misc.py

@@ -112,6 +112,14 @@ def find_subclasses(locations, base_cls, exclude_hidden=True):
     return derived
 
 
+def pick_first_not_none(*values):
+    """Returns first of values that is *not* None (or None if all are/were)."""
+    for val in values:
+        if val is not None:
+            return val
+    return None
+
+
 def parse_uri(uri):
     """Parses a uri into its components."""
     # Do some basic validation before continuing...