test_volume_encrypted: Stop creating an image for each instance

An independent image isn't required for these tests and additionally
isn't required when calling create_server as the lack of an image
results in CONF.compute.image_ref being used by
tempest.common.compute.create_test_server [1].

The creation of this image resulted in the nova-ceph-multistore job
breaking as documented in bug #1912607. This broke as we use the job to
provide both our core ceph coverage and slightly more edgey topologies
using Glance multistore and the [workarounds]/ workaround option. This
option blocking the download of rbd images via g-api when n-cpu can't
clone the rbd volumes directly, as caused by the image creation in
test_volume_encrypted.

Iaf6f6e0dbcb25561bf00e969e1964cd30e974e64 is currently skipping any
encryption test in the nova-ceph-multistore job to workaround bug
 #1912607 for the time being but we would obviously like to revert that
if possible and expand our coverage.

A follow up change will also propose adding nova-ceph-multistore to the
gate of this plugin to avoid future breakage.

Related-Bug: #1912607

[1] https://opendev.org/openstack/tempest/src/branch/master/tempest/common/compute.py#L108-L109

Change-Id: I50d3b874b7e0676436c434be6c059f221041560f
1 file changed