Allow network configuration for mtu tests

In NFV baremetal deployments, networks settings may be constrained.
This patch allows to configure on create:
 subnet cidr, for provider networks not managed by neutron
 network type, VLAN/VXLAN/GRE/FLAT
 mtu, network mtu for OVS+DPDK bridge/bond mtu settings.

It accommodate network environment in class NetworkWritableMtuTest.
It also allows to test ovs [through ovs+dpdk] jumbo and mtu.

Change-Id: Ia519d3bb7eb32b1f986983c9b74ee3621881172f
2 files changed