NetworkAgent: correctly identify the VPN secret requests

NetworkAgent: correctly identify the VPN secret requests

The non-interactive requests for 'vpn' settings are forwarded to the UI because
it is able to talk to the auth helpers. However, the VPN requests are identified
by the connection type instead of setting type. That is incorrect and the UI
is not prepared to handle such requests; tries to construct a dialog and fails
miserably:

  Gjs-Message: JS LOG: Invalid connection type: vpn

  (gnome-shell:13133): Gjs-WARNING **: JS ERROR: Error: No property 'text' in property list (or its value was undefined)
  NetworkSecretDialog<._init@resource:///org/gnome/shell/ui/components/networkAgent.js:60
  wrapper@resource:///org/gnome/gjs/modules/lang.js:169
  _Base.prototype._construct@resource:///org/gnome/gjs/modules/lang.js:110
  Class.prototype._construct/newClass@resource:///org/gnome/gjs/modules/lang.js:204
  NetworkAgent<._handleRequest@resource:///org/gnome/shell/ui/components/networkAgent.js:724
  wrapper@resource:///org/gnome/gjs/modules/lang.js:169
  NetworkAgent<._newRequest@resource:///org/gnome/shell/ui/components/networkAgent.js:715
  wrapper@resource:///org/gnome/gjs/modules/lang.js:169

https://bugzilla.gnome.org/show_bug.cgi?id=760999