[AIRFLOW-6060] Improve conf_vars context manager (#6658)

This commit adds try / finally clause to conf_vars context
manager to assure that initail values are reseted in case
of an exception in yield.

tests/test_utils/config.py

@@ -28,7 +28,6 @@ from airflow.configuration import conf
 def conf_vars(overrides):
     original = {}
     original_env_vars = {}
-    reconfigure_vars = False
     for (section, key), value in overrides.items():
 
         env = conf._env_var_name(section, key)
@@ -43,12 +42,10 @@ def conf_vars(overrides):
             conf.set(section, key, value)
         else:
             conf.remove_option(section, key)
-
-        if section == 'core' and key.lower().endswith('_folder'):
-            reconfigure_vars = True
-    if reconfigure_vars:
     settings.configure_vars()
+    try:
         yield
+    finally:
         for (section, key), value in original.items():
             if value is not None:
                 conf.set(section, key, value)
@@ -56,5 +53,4 @@ def conf_vars(overrides):
                 conf.remove_option(section, key)
         for env, value in original_env_vars.items():
             os.environ[env] = value
-    if reconfigure_vars:
         settings.configure_vars()