Hide [notification] smtp_user_password from debug logs

The option accepts a credential.

Change-Id: Ia4d6313b653df36f3deff7f05828ef75171a1c15
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

zaqar/conf/notification.py

@@ -45,6 +45,7 @@ smtp_user_name = cfg.StrOpt(
 
 smtp_user_password = cfg.StrOpt(
     'smtp_user_password',
+    secret=True,
     help='The user password for the email system to login. It '
          'should be set when smtp_mode is set to self_local.')