IPv6 job using UEFI and OVN with dhcpv6-stateful address mode.
Updates the devstack plugin to ensure CentOS DIB ironic-python-agent is
always used for dhcpb6-stateful, udhcpc in tinycore does not support
DHCPv6.
Ensure mtu on the ironic-provision network matches PUBLIC_BRIDGE_MTU
when Ironic IPv6 is used. This ensures we do not get packet drops from
over-mtu.
Devstack plugin will ignore any HOST_IPV6 address discovered, always
using the magigv6 interface and 'fc00::1' as IRONIC_HOST_IPV6.
Change-Id: Iab97d78d7a075eaef3bdcfc08fc4f184a5ea490a
782045852f