engine: Changing mtu to default keeps old MTU value on the host

Changing mtu of the network to default should override the current mtu
of the network on the host (or at least mark it as unsync).

Change-Id: I327bafad911c207c45e0250b43c297a05114dc1a
Bug-Url: https://bugzilla.redhat.com/1084961
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>