iproute2: raise ConfigNetworkError when dhcp fails and blockingdhcp is set

It is important that a ConfigNetworkError exception is raised when
doing the addressing synchronously, otherwise it fails silently.

Change-Id: I6bbcd0ccee2371413e228c9709cc1df984329348
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28559
Reviewed-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>