nmcli/dev: use secret agent for nmcli d [wifi] connect

Instead of asking the Wi-Fi password in advance (or not at all, if we're creating a new connection for "nmcli d conn"), use the secret agent.

This makes things consistent with other places where we handle the secrets for an activating connection in nmcli ("nmcli c up", "nmcli d con" with an existing connection).

This also fixes the situation where the secrets would stop being required, such as on enrollment via WPS button press on a router.