Comment 8 for bug 1757407

Reviewed: https://review.openstack.org/584969
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=59075b8243e869774e29bd3b378723947c0b3a20
Submitter: Zuul
Branch: stable/queens

commit 59075b8243e869774e29bd3b378723947c0b3a20
Author: Balazs Gibizer <email address hidden>
Date: Thu Apr 26 16:55:15 2018 +0200

    Call generate_image_url only for legacy notification

    The legacy instance.exists notification includes the full url of the glance
    image of the given instance. But the versioned notification only includes
    the image uuid. Generating the full url can be a costly operation as it
    needs to talk to Keystone.

    So this patch makes sure that generate_image_url only called when the
    generated information will be used.

     Conflicts:
     nova/compute/utils.py
     nova/notifications/base.py
     nova/tests/unit/notifications/test_base.py

    NOTE(elod.illes): conflict caused by parameter system_metadata which is not
    there on master anymore.

    Change-Id: I78c2a34b3d03438457cc968cd0a38b8131e4f6e6
    Closes-Bug: #1757407
    (cherry picked from commit 93b897348bde072969f67e43875ce08e5d420b8a)