global_physnet_mtu needs to be set in neutron.conf, because linuxbridge-agent
discovers underlying vxlan0 interface mtu and returns an error when creating
vxlan port

CentOS8 job will not be added, because CentOS 8 iptables-ebtables package
is missing broute (--among-src) tables support required for linuxbridge agent,
see [1].

[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1720637

Change-Id: I6b12f7ba95401d3342359c57ceeee8bec8aefe49