engine: remove Config dependency from frontend

The only use of class org.ovirt.engine.core.common.config.Config is to
get the default configuration version.

Replacing it with a static const in the client, which is
fetch from server side using a new query during client init.

Change-Id: I47e6f47bff95ca630f192c0b3197688c5caf2948
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>