Fix duplicated images in test_block_device_mapping

In test_block_device_mapping, each test can select to specify
image_ref, block_device_mapping or both. The "both" should be
negative tests because nova API gets image_id from either image_ref
or block_device_mapping.
However, there are some "both" tests which are not their purposes.
This patch fixes these tests for their purposes by passing True as
no_image.

These bugs were found through jsonschema validation framework
development, because the schema could detect "both" cases fastly
and these tests could not get their purposes.

Change-Id: I73f1a15d8e08a3eda3190e660338e274358ddd33
Merge "Fix duplicated images in test_block_device_mapping"