Review https://review.openstack.org/#/c/540077/ setting
requires global_physnet_mtu to be set lower than the
underlying network's mtu. So in some cases, For example
vm on ophenstack cloud by default has 1450 as mtu, so
global_physnet_mtu for neutron deployed on these vm should
be set to <= 1450.
Change-Id: Ib305f0ecfd8402dfe16f84eda21928fd2e615280