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/nginx.py

@@ -55,5 +55,5 @@ class Nginx(base.Installer):
         system.add_user_to_group(user, group)
 
         if not os.path.exists("{}/dhparam.pem".format(self.config_dir)):
-            cmd = "openssl dhparam -out dhparam.pem 4096"
+            cmd = "openssl dhparam -dsaparam -out dhparam.pem 4096"
             utils.exec_cmd(cmd, cwd=self.config_dir)