linuxbridge agent and l3agent: missing options in logs, need to log config earlier

[Thomas Morin]

https://review.openstack.org/#/c/504457 fixes the OVS agent behavior so that all config options, including the one registered by L2 agent extensions, are logged on startup.

This bug is to track the fact that the linuxbridge and l3agent havn't yet been fixed.

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: stable/pike
Review: https://review.openstack.org/508277

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/508277
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=3fc09ed05a1b3613c5fbbc6f955dbf68889e9a3e
Submitter: Jenkins
Branch: stable/pike

commit 3fc09ed05a1b3613c5fbbc6f955dbf68889e9a3e
Author: Ihar Hrachyshka <email address hidden>
Date: Fri Sep 15 10:46:26 2017 -0600

    ovs: log config options when all of them are registered

    Otherwise we don't see some of them for the agent, for example,
    AGENT.root_helper is missing.

    To make sure the logging is as early as possible, and to make sure that
    options that may be registered by extensions are also logged, some
    refactoring was applied to the code to move the extension manager
    loading as early as possible, even before agent's __init__ is called.

    Related-Bug: #1718767
    Change-Id: I823150cf6406f709d1e4ffa74897d598e80f5329
    (cherry picked from commit 45be804b4069599e8eda9820084fcf11fb94167b)