[OVN] Run maintenance task whenever the OVN DB schema has been upgraded

[Daniel Alvarez]

When OVN DBs are upgraded (and restarted), there might be cases whenever we want to accommodate things to a new schema. In this situation we don't want to force a restart of neutron-server (or metadata agent) but instead, detect it and run whatever is needed.

This can be achieved by checking the schema version via ovsdbapp [0] and comparing if it's bigger than what we had upon a reconnection to the OVN DBs.

[0] https://github.com/openvswitch/ovs/blob/master/python/ovs/db/schema.py#L35

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/709770

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/709770
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=9e416224b04d7d4094e5719afa4ef5bccbe48ce7
Submitter: Zuul
Branch: master

commit 9e416224b04d7d4094e5719afa4ef5bccbe48ce7
Author: Lucas Alvares Gomes <email address hidden>
Date: Tue Feb 25 15:14:54 2020 +0000

    [OVN] OVN DB schema update aware maintenance tasks

    This patch introduces a new mechanism to allow rerunning maintenance
    tasks upon an OVN database schema change to avoid a service restart.

    As an example, the "migrate_to_port_groups" maintenance task will run
    again when the database schema is updated. In case of a migration from
    an OVN version without port groups support to a version that supports
    it, the OVN driver will migrate the code automatically without the need
    of a service restart.

    Closes-Bug: #1864641
    Change-Id: I520a3de105b4c6924908e099a3b8d60c3280f499
    Signed-off-by: Lucas Alvares Gomes <email address hidden>