Comment 8 for bug 1816771

Reviewed: https://review.openstack.org/638380
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=d802fad8a92625005597ebda4931b0bbe13418e9
Submitter: Zuul
Branch: master

commit d802fad8a92625005597ebda4931b0bbe13418e9
Author: Slawek Kaplonski <email address hidden>
Date: Thu Feb 21 11:16:03 2019 +0100

    Avoid loading same service plugin more than once

    In patch [1] requirement that only each service plugin
    can be loaded only once was removed.
    Unfortunatelly it is not possible that same service plugin
    will be instantiate more than once because it may reqister some
    callbacks or other things which can't be duplicated.

    So this patch adds mechanism which will ensure that each service
    plugin class is instantiate only once and reused if necessary.

    [1] https://review.openstack.org/#/c/626561/

    Closes-Bug: #1816771

    Change-Id: Ie6e6cc1bbbe50ff7cfad4e8033e48711569ea020