Unnecessary BDM entry is created when attempting to attach the same volume to an instance twice.

Detailed bug description:
An unnecessary block device mapping (BDM) entry is created in the `block_device_mapping` table of Nova whenever attempting to attach the same volume to an instance twice.

Steps to reproduce:

1. Create a standalone instance.
2. Create a volume in Cinder.
3. Attach the volume to the instance using the 'nova volume-attach' command.

# nova volume-attach <instance-id> <volume-id>

4. Confirm the volume was attached; this can be seen in the 'nova show' output.

# nova show <instance-id>

5. Execute the command from Step #3 to attempt to attach the volume once more. Nova will produce an "Invalid volume" error.

ERROR (BadRequest): Invalid volume: volume <volume-id> status must be 'available'. Currently in 'in-use' (HTTP 400) (Request-ID: req-xxxx)

Expected results:

Nova will throw the error listed above, and not create any additional BDM entries in the Nova database.

Actual result:

Nova throws the error, but proceeds with creating a NULL BDM entry. This can be seen by executing the following MySQL query:

SELECT * FROM block_device_mapping WHERE instance_uuid='XXXX';

Reproducibility:

** See upstream bug: https://bugs.launchpad.net/nova/+bug/1427060

I'm able to reproduce on MOS 8.0.

[root@fuel ~]# fuel fuel-version
api: '1'
auth_required: true
feature_groups:
- mirantis
openstack_version: liberty-8.0
release: '8.0'