netconf: Fix the mtu of bond/nic not being set for new network

The configuring should not be skipped if the new vlaned network
over the bond or nic has a bigger mtu than current.

This patch fixes the testSetupNetworksMtus functional test, broken by
http://gerrit.ovirt.org/17491.

Bug-Url: https://bugzilla.redhat.com/1004196
Change-Id: Id7a1f5b628261ae4ebf400494bcdb2939a9cd534
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18966
Reviewed-by: Assaf Muller <amuller@redhat.com>