manager: change autoconnect-slaves logic for already active slaves

Autoconnect-slaves currently forces an activation of all slaves, even if there is already an active connection for them. This is bad because at boot slaves first try to autoconnect, then the autoconnect-slaves of the master kicks in and disconnects/reactivates them.

The only reason why the forceful reactivation was added was to fix [1]; in that scenario, a slave connection is already active as non-slave; then it is updated to be a slave; later, the master with autoconnect-slaves is manually activated. NetworkManager should detect that the slave connection must now be activated by autoconnect-slaves.

Add a specific check for such situation, instead of always reactivating all slaves.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1548265

Fixes: 4985ca5a ('manager: allow autoconnect-slaves to reconnect the same connection')