Fix receiver feedin to email

Email receiver only accept string. Make sure we convert to string.
diff --git a/staffeln/conductor/result.py b/staffeln/conductor/result.py
index c185d5e..6d48093 100644
--- a/staffeln/conductor/result.py
+++ b/staffeln/conductor/result.py
@@ -33,11 +33,11 @@
             subject = "Staffeln Backup result"

         if len(CONF.notification.receiver) != 0:

             # Found receiver in config, override report receiver.

-            receiver = CONF.notification.receiver

+            receiver = ",".join(CONF.notification.receiver)

         elif not CONF.notification.project_receiver_domain:

             try:

-                receiver = self.backup_mgt.openstacksdk.get_project_member_emails(

-                    project_id

+                receiver = ",".join(

+                    self.backup_mgt.openstacksdk.get_project_member_emails(project_id)

                 )

                 if not receiver:

                     LOG.warn(