revise auth file path computation

Author: d-w-moore

irods/__init__.py

@@ -13,8 +13,11 @@ def env_filename_from_keyword_args(kwargs):
             env_file = os.path.expanduser('~/.irods/irods_environment.json')
     return env_file
 
-def derived_auth_filename(env_file):
-    return '' if not env_file else os.path.join(os.path.dirname(env_file),'.irodsA')
+def derived_auth_filename(env_filename):
+    if not env_filename:
+        return ''
+    default_irods_authentication_file = os.path.join(os.path.dirname(env_file),'.irodsA')
+    return os.environ.get('IRODS_AUTHENTICATION_FILE', default_irods_authentication_file)
 
 # This has no effect if basicConfig() was previously called.
 logging.basicConfig()