Make body of std.email optional

According to SMTP protocol both the subject and body of the email
can be blank. Hence, making the body of std.email as optional.

Change-Id: I1a2551467247a46c4e3fa5425a367d0f83756b9c
Closes-Bug: #1649366

mistral/actions/std_actions.py

@@ -282,7 +282,7 @@ class SendEmailAction(base.Action):
         # Task invocation parameters.
         self.to = to_addrs
         self.subject = subject or "<No subject>"
-        self.body = body
+        self.body = body or "<No body>"
 
         # Action provider settings.
         self.smtp_server = smtp_server