Comment 13 for bug 1816771

Reviewed: https://review.openstack.org/649043
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=fb0c062899032288091df50a18f7f30a594bcc14
Submitter: Zuul
Branch: stable/rocky

commit fb0c062899032288091df50a18f7f30a594bcc14
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
    (cherry picked from commit d802fad8a92625005597ebda4931b0bbe13418e9)