Fix Webmail [AUTHENTICATIONFAILED] on MySQL

Fix Webmail (['[AUTHENTICATIONFAILED] Authentication failed.']) with MySQL database.

modoboa_installer/scripts/dovecot.py

@@ -50,8 +50,7 @@ class Dovecot(base.Installer):
 
     def post_run(self):
         """Additional tasks."""
-        if self.dbengine != "postgres":
-            return
+        if self.dbengine == "postgres":
             dbname = self.config.get("modoboa", "dbname")
             dbuser = self.config.get("modoboa", "dbuser")
             dbpassword = self.config.get("modoboa", "dbpassword")