engine: added spice proxy to vds and vm_static

This patch adds the spice_proxy field to vds and to
vm_pools.

The meaning of this field is to give the possibility to override the
SpiceProxyDefault configuration from vdc_options on two different levels
(cluster and pool).

Change-Id: I5dd4f5ef908464d75af6911eedf2dd053d07cb9e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=974076
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>