Comment 71 for bug 259214

Maybe I found workaround to disable automatic connection for default system connections.
run nm-connection-editor from root:
sudo nm-connection-editor
you will gain root access to system connections, so you can disable autoconnect for them.
user's connections are not accessible from root, so there is no way to make them system this way.

still if user's connection has autoconnect, but system connections do not, then user's must connect first, I presume. Will see on next boot...