fix(server.conf): use user & group nobody instead

We've been using using *openvpn* user and group as server launch
config options.

But latest versions of the popular distros are packaging OpenVPN
to be used with *nobody* user and group and they are not creating
openvpn user anymore, as also stated in #47.

Signed-off-by: Mustafa Arici <mustafa@arici.io>

template/server.conf.tmpl

@@ -248,8 +248,8 @@ comp-lzo
 #
 # You can uncomment this out on
 # non-Windows systems.
-user openvpn
-group openvpn
+user nobody
+group nobody
 
 # The persist options will try to avoid
 # accessing certain resources on restart