merge: enable VPN secrets for nmtui/nmcli secret agent (rh #975185)

It allows nmcli and nmtui to ask for VPN passwords and thus successfully
activate VPN connections.
There is a special handling for OpenConnect, because the user does not know
the secrets. They have to be obtained by authenticating to OpenConnect server.

https://bugzilla.redhat.com/show_bug.cgi?id=975185