restapi: Allow enabling/disabling SSO

This small feature allows controlling SSO per VM via REST API.

Change-Id: I25bf94646ea89684152e48f25ad604db6e59e86c
Bug-Url: https://bugzilla.redhat.com/1035279
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>