test_volume_multiattach.VolumeMultiattachTests.test_multiattach_data_integrity depends on encrypted volumes without declaring the dependency

[Luigi Toscano]

cinder_tempest.plugin.scenario.test_volume_multiattach.VolumeMultiattachTests.test_multiattach_data_integrity creates an encrypted volume and it attaches it to the instance.
According the comments in the review which added the test, the idea was to simulate a real scenario and use an instance with multiple attached volumes while performing some operations on additional attached multiattach volumes.

Unfortunately this means that the test can't work when encryption is not supported.

As the test is valuable even when encryption is not supported and disabling it is not wanted, I see two possible ways:

- just use a simple volume (not encrypted) when encryption is not supported
- never use encrypted volumes, but just defined another type of volume (with random properties) instead