Make OpenSSL use less computing power, closes #87

http://security.stackexchange.com/questions/95178/diffie-hellman-parameters-still-calculating-after-24-hours suggests to add the `-dsaparam` to the OpenSSL commandline for unarbitrarily long running Diffie-Hellman parameters generation.

modoboa_installer/scripts/postfix.py

@@ -85,5 +85,5 @@ class Postfix(base.Installer):
 
         # Generate EDH parameters
         if not os.path.exists("{}/dh2048.pem".format(self.config_dir)):
-            cmd = "openssl dhparam -out dh2048.pem 2048"
+            cmd = "openssl dhparam -dsaparam -out dh2048.pem 2048"
             utils.exec_cmd(cmd, cwd=self.config_dir)