Make SMTP authentication mechanism configurable.

config/discourse_defaults.conf

@@ -56,6 +56,9 @@ smtp_user_name =
 # password for smtp server
 smtp_password =
 
+# smtp authentication mechanism
+smtp_authentication = plain
+
 # enable TLS encryption for smtp connections
 smtp_enable_start_tls = true
 

config/environments/production.rb

@@ -39,7 +39,7 @@ Discourse::Application.configure do
       domain:               GlobalSetting.smtp_domain,
       user_name:            GlobalSetting.smtp_user_name,
       password:             GlobalSetting.smtp_password,
-      authentication:       'plain',
+      authentication:       GlobalSetting.smtp_authentication,
       enable_starttls_auto: GlobalSetting.smtp_enable_start_tls
     }