webadmin,userportal: set up Proxy property to SPICE

This patch reads the SpiceProxyDefault property from vdc_options
and sets it to the Proxy property of SPICE client(both cab and xpi).

If the SpiceProxy is not set then nothing is set to the client.

Change-Id: If269869413eae577fb243bffe849548f90ff8f4a
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>