std.email action requires a smtp_password

When attempting to run a workbook with an email action (std.email)
you have to specify a value for smtp_password.This patch attempts
to fix this bug.

Change-Id: I60f30e306ba673a1015e4bfb156ce282ed98406a
Closes-Bug: #1639682

mistral/actions/std_actions.py

@@ -276,7 +276,7 @@ class MistralHTTPAction(HTTPAction):
 
 class SendEmailAction(base.Action):
     def __init__(self, from_addr, to_addrs, smtp_server,
-                 smtp_password, subject=None, body=None):
+                 smtp_password=None, subject=None, body=None):
         # TODO(dzimine): validate parameters
 
         # Task invocation parameters.

mistral/tests/unit/services/test_action_manager.py

@@ -32,7 +32,7 @@ class ActionManagerTest(base.DbTestCase):
 
         std_email_input = (
             "from_addr, to_addrs, smtp_server, "
-            "smtp_password, subject=null, body=null"
+            "smtp_password=null, subject=null, body=null"
         )
 
         self.assertEqual(std_email_input, std_email.input)