Add a new variant needle for GNOME reboot confirm screen

Seems none of the existing needles matches on current F28 update
tests.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

needles/gnome/gnome_reboot_confirm-f28-20181011.json

@@ -0,0 +1,16 @@
+{
+  "tags": [
+    "DESKTOP-gnome",
+    "gnome_reboot_confirm"
+  ],
+  "area": [
+    {
+      "type": "match",
+      "ypos": 517,
+      "width": 110,
+      "height": 18,
+      "xpos": 588
+    }
+  ],
+  "properties": []
+}