[OVN] Add baremetal support without Neutron DHCP agent for IPv4

This patch adds support for deploying baremetal nodes with OVN's
built-in DHCP server for IPv4.

Since Neutron API's for setting DHCP options is mostly a pass-thru,
Ironic uses a dnsmasq syntax for setting the baremetal options [0].
Since this syntax is unlikely to change and it's only a tiny subset of
what dnsmasq can offer this patch does translate that syntax used by
Ironic and convert it to OVN's equivalent options. In this way we do not
need to re-design Neutron's DHCP options API nor change Ironic to use it
with ML2/OVN.

This option also adds a new configuration option called
"disable_ovn_dhcp_for_baremetal_ports". PXE booting nodes can be very
sensitive and operators may prefer to use a fully-fledged DHCP server to
do it (even Ironic makes DHCP pluggable). So if operators wish to
disable OVN's built-in DHCP server for baremetal provisioning they can
do so by setting this new option to True. It defaults to False.

This change has been tested with real hardware and it does work. That
said, we found a problem in core OVN itself [1] while testing it that
can affect PXE from reaching the TFTP server, we already communicated
this with the core OVN folks and we hope it can be fixed soon. The
change in core OVN should not affect the Neutron change tho.

Not that the "server-ip-address" DHCP Option now points to the
"next_server" option in OVN instead of the "tftp_server_address". The
previous behavior was wrong, the "server-ip-address" should set the
"siaddr" in the DHCP header and this has been introduced in OVN [2] as
an option called "next_server".

[0]
https://github.com/openstack/ironic/blob/49113385e89c52b56152418d3a0c8c69ddaf8b6e/ironic/common/pxe_utils.py#L523-L538
[1]
https://mail.openvswitch.org/pipermail/ovs-discuss/2022-May/051821.html
[2]
https://patchwork.ozlabs.org/project/ovn/patch/20220511142757.168196-1-lmartins@redhat.com/

Partial-Bug: #1971431
Change-Id: Ia041f640293ba26abf9f70af915817e9861e8ffc
Signed-off-by: Lucas Alvares Gomes <lucasagomes@gmail.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/releasenotes/notes/ovn-baremetal-provisioning-04211d04d4897833.yaml b/releasenotes/notes/ovn-baremetal-provisioning-04211d04d4897833.yaml
new file mode 100644
index 0000000000..82ada6af52
--- /dev/null
+++ b/releasenotes/notes/ovn-baremetal-provisioning-04211d04d4897833.yaml
@@ -0,0 +1,14 @@
+---
+features:
+  - |
+    Support for baremetal provisioning using OVN's built-in DHCP server
+    has been added for IPv4.
+upgrade:
+  - |
+    A new configuration option called
+    ``[ovn]/disable_ovn_dhcp_for_baremetal_ports`` has been added to
+    ML2/OVN for IPv4. Since PXE booting nodes can be very sensitive
+    depending on the hardware and some operators may prefer to use a
+    fully-fledged DHCP server instead of OVN's DHCP server this option
+    allows for disabling OVN's built-in DHCP server for baremetal ports
+    (vnic type "baremetal") when set to True. It defaults to False.