tools : engine-config should return the list of possible VNC keyboard layouts

Changing the default VNC keyboard layout is possible via engine-config.
There should be an indication to what are the possible values.

The information on the possible layouts is stored at vdc_options table
(VncKeyboardLayoutValidValues option name)

Change-Id: I33c37b22501c64cad694e28941bd5df181bbbe6f
Bug-Url: https://bugzilla.redhat.com/987952
Signed-off-by: Ravi Nori <rnori@redhat.com>