Add openconnect usergroup parameter support

Openconnect supports passing usergroup as parameter for VPN connections. Some providers are offering different sign-in pages hostet on different url root paths for controlling usergroups or realms. E.g. myvpn.host.com/usergroup1, myvpn.host.com/realm2.

When configuring VPN openconnect profiles with NM (e.g. gnome), usergroup/realm information gets lost after auth dialog because only FQDN is forwarded to openconnect service resulting in 404 HTTP error.

Add a new parameter/setting OpenConnect VPN GUI Form field Usergroup. Parameter stored to VPN setting NM-connection. Parameter, if not empty, will be passed to openconnect service and used to call binary with argument --usergroup=value.

Possible fix for: #18