Change: I11e38f82eb9040f77412fe8ad200fcc48031e2f8 introduced mtu
property for composable networks. This change set the MTU of the
Tenant network as the global_physnet_mtu for neutron, unless the
NeutronGlobalPhysnetMtu is overridden. The default MTU used if
no MTU is defined for the Tenant network is 1500. (The same
default was previously used for the NeutronGlobalPhysnetMtu
parameter.)
Change-Id: I5e60d52ad571e1cdb3b82cd1d9947e33fa682bf8