Add Tempest tests for LVM ephemeral storage encryption

 LVM ephemeral storage encryption feature was added to Nova in Juno, but Tempest testing was not possible at the time because DevStack did not have the capability to stand up Nova with LVM ephemeral storage. This has now been fixed, so the feature should be tested as was originally proposed in its blueprint.

The tests will include simple encryption tests, such as verifying that a test file created from within the instance is not readable on the compute host's disk. In addition, all of the existing Tempest tests applicable to LVM backed ephemeral instance should also be valid for encrypted LVM instances.