nova instance snapshot does not wait for volume snaps

Currently when snapping a boot-from-volume instance nova immediately returns success and glance reports active image. Snapping of disks can take some time and if we try to launch a new instance from that image as soon as we get active image state we get error. Nova should wait until all snaps are ready before returning success.

relevant code lines:
https://github.com/openstack/nova/blob/512fbdfa9933f2e9b48bcded537ffb394979b24b/nova/compute/api.py#L3445

because in my backend setup snaps are somewhat long I always fail multiattach tempest test:

{0} tearDownClass (tempest.api.compute.volumes.test_attach_volume.AttachVolumeMultiAttachTest) [0.000000s] ... FAILED

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/opt/stack/tempest/tempest/test.py", line 220, in tearDownClass
    raise value.with_traceback(trace)

      File "/opt/stack/tempest/tempest/test.py", line 192, in tearDownClass
    teardown()

      File "/opt/stack/tempest/tempest/test.py", line 602, in resource_cleanup
    raise testtools.MultipleExceptions(*cleanup_errors)

    testtools.runtest.MultipleExceptions: (<class 'tempest.lib.exceptions.BadRequest'>, Bad request
Details: {'code': 400, 'message': 'Invalid volume: Volume status must be available or error or error_restoring or error_extending or error_managing and must not be migrating, attached, belong to a group, have snapshots, awaiting a transfer, or be disassociated from snapshots after volume transfer.'}, <traceback object at 0x7f622cbbdc80>)

This happens because we try to delete boot volume and snap while snap is not ready. Also this trace is while trying to create a new instance from 'active' image:

юл 04 11:13:10 openstack-dev-00 nova-compute[1034924]: ERROR nova.compute.manager [None req-8122e11f-b490-4c68-b447-e35c3ad21cdf demo admin] [instance: 10cefe2f-ac44-4cb8-9006-28443481ba9d] Build of instance 10cefe2f-ac44-4cb8-9006-28443481ba9d aborted: Invalid input received: Invalid snapshot: Originating snapshot status must be one of 'available' values (HTTP 400) (Request-ID: req-32350814-f125-448f-a620-dc8526e6256b): nova.exception.BuildAbortException: Build of instance 10cefe2f-ac44-4cb8-9006-28443481ba9d aborted: Invalid input received: Invalid snapshot: Originating snapshot status must be one of 'available' values (HTTP 400) (Request-ID: req-32350814-f125-448f-a620-dc8526e6256b)