frontend: Make VNC implementation configurable from UI

This patch enables configuring VNC client mode to be configurable from
the console popup dialog.

Currently two options are available:
 - Native Client (MIME)
 - noVNC.

The patch also adds persisting the selected value to the browser
storage.

Change-Id: Ied777eaefe6c6dc25a002e761bbf5b501710b2ae
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>