engine: Send defRoute by VDSM-reported capabilities

Previously it was sent based on the engine's cluster compatibility
version, which in the case of a < 3.4 host in >= 3.4 cluster
(non-operational but can still receive Setup Networks command) would
cause an exception in VDSM.

Change-Id: I7d8b608e9f68a874daf11e2ae73280fe9961c93d
Bug-Url: https://bugzilla.redhat.com/1085838
Signed-off-by: Lior Vernia <lvernia@redhat.com>