frontend: Include ticket validity time in .vv file

Include "# Ticket is valid for XYZ seconds." in .vv file for SPICE and
VNC.

Change-Id: I821d1a0081f2755ac2d55094471247a1fc7ba7f5
Bug-Url: https://bugzilla.redhat.com/977306
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>