restapi: added display.proxy property to vmpool and cluster

Originally it was only possible to set the SPICE proxy up in system level
(SpiceProxyDefault from vdc_options). This adds the way to override it on
cluster or vmpool level using display.proxy property.

Change-Id: I3ba32d8bdc9fc790dc9ca4853a01a1d9f2d08661
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=974076
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>