feature: allow 'dvr_snat' l3 agent mode to be used instead of just 'dvr' to allow for neutron-gateway-less deployments

[Dmitrii Shcherbakov]

Currently it is a requirement to have a network node with an l3 agent running in the dvr_snat mode even for DVR deployments that do not use SNAT or have a very limited usage of SNAT.

It is not possible to disable snat completely: https://bugs.launchpad.net/neutron/+bug/1761591

Neutron creates a network:router_centralized_snat port and if it is not possible to find a dvr_snat agent to schedule it on there are various side-effects which are not seen at first. For example, Designate stops creating records for floating IPs and Neutron/Designate integration is, therefore, not functional.

Functionality relevant to dvr_snat:

* nodes that host l3 agents in dvr_snat mode are used for network:router_centralized_snat port bindings (otherwise there is nowhere to bind this port and it stays in the DOWN state);
* l3 agents in dvr_snat mode create snat-<router-uuid> namespaces that have rules perform port address translation functionality;
* l3 agents in dvr_snat mode are placeholders for parts of an L3HA router.

The documentation says that dvr_snat should be used on network nodes:
https://docs.openstack.org/neutron/queens/admin/deploy-ovs-ha-dvr.html#network-node

However, there is nothing restricting a DVR deployment from using dvr_snat l3 agents on every compute node and not having dedicated network nodes.

https://wiki.openstack.org/wiki/Neutron/DVR/HowTo (example: devstack environment)

Rationale: for deployments that only need FIPs with DVR or are known to have low SNAT traffic usage requirements it makes sense not to have network nodes. Therefore, neutron-openvswitch can be extended to optionally use dvr_snat mode.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/624495

[Dmitrii Shcherbakov (dmitriis)]

Subscribed ~field-medium as this is needed for ongoing field projects.

[James Page (james-page)]

Worth noting that the n-gateway units also do some other tasks such as lbaas (for pre-octavia deployments) and metering; these are not currently included in the scope of the neutron-openvswitch DVR support. Worth checking on the fwaas support in the l3-agent for n-ova as well.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/624495
Committed: https://git.openstack.org/cgit/openstack/charm-neutron-openvswitch/commit/?id=1486c83a1f15e39cb7c4233996fec6668c663949
Submitter: Zuul
Branch: master

commit 1486c83a1f15e39cb7c4233996fec6668c663949
Author: Dmitrii Shcherbakov <email address hidden>
Date: Wed Dec 12 00:12:21 2018 +0300

    Allow dvr_snat l3 agent mode to be used with DVR

    Currently it is a requirement to have a network node with an l3 agent
    running in the dvr_snat mode even for DVR deployments that do not use
    SNAT or have a very limited usage of SNAT.

    It is not possible to disable snat completely:
    https://bugs.launchpad.net/neutron/+bug/1761591

    Neutron creates a network:router_centralized_snat port and if it is not
    possible to find a dvr_snat agent to schedule it on there are various
    side-effects which are not seen at first. For example, Designate stops
    creating records for floating IPs and Neutron/Designate integration is,
    therefore, not functional.

    The Neutron DVR documentation says that dvr_snat should be used on
    network nodes. However, there is nothing restricting a DVR deployment
    from using dvr_snat l3 agents on every compute node and not having
    dedicated network nodes.

    This change modifies neutron-openvswitch to optionally enable dvr_snat
    l3 agent mode (this includes supporting L3HA routers if enabled). As a
    result, it is possible to have deployments without neutron-gateway thus
    saving on the amount of required nodes. Care should be taken when a
    large amount of L3HA routers is used and using DVR routers without L3HA
    is a recommended.

    Change-Id: Iad3a64967f91c81312911f6db856ce2271b0e068
    Closes-Bug: #1808045