config_drive is always enabled for amphora VMs [1], so DHCP is not
required for cloud-init to configure octavia network interface.
To avoid confusion, this patch disables DHCP for octavia network by default.
[1] eff69261f4/octavia/compute/drivers/nova_driver.py (L146)
Change-Id: Iabb1afac5dcea64251813404d924da9a648ca2e9