gdm: Refactor oVirt to a generic CredentialManager interface

Commit 4cda61a1 added support for pre-authenticated logins in
    oVirt environments. This feature prevents a user from having
    to type their password twice (once to the oVirt management machine,
    and then immediately again in the provisioned guest running gnome-shell).
    That feature is currently oVirt specific, but a similar feature would
    be useful in non-oVirt based virt farm environments.

    Toward that end, this commit generalizes the various aspects of the
    oVirt integration code, so that it can be reused in a subsequent
    commit for adding single sign on support in vmware deployments, too.

    Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/1983

js/gdm/credentialManager.js

@@ -0,0 +1,24 @@
+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+/* exported CredentialManager */
+
+class CredentialManager {
+    constructor(service) {
+        this._token = null;
+        this._service = service;
+        this._authenticatedSignalId = null;
+    }
+
+    get token() {
+        return this._token;
+    }
+
+    set token(t) {
+        this._token = t;
+        if (this._token)
+            this.emit('user-authenticated', this._token);
+    }
+
+    get service() {
+        return this._service;
+    }
+}