Should not allow duplicate device name when provide bdm for create

[Zhenyu Zheng]

Currently when we create instance with bdm provided, if multiple bdm is using the same device name, for example, all of them are vdb, the latter bdm will override the previous
bdms and it will lead to only one bdm created.

We should raise error for this kind of scenario.

A test log can be found in:
http://paste.openstack.org/show/591749

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/408475

[OpenStack Infra (hudson-openstack)]

Change abandoned by Zhenyu Zheng (<email address hidden>) on branch: master
Review: https://review.openstack.org/408475

[Matt Riedemann (mriedem)]

According to the comments from Feodor on https://review.opendev.org/#/c/408475/ this is a valid bug but the solution needs work, so I'm going to mark this as confirmed but we'd likely want a functional recreate of the issue first and then a fix to go on top and show it's working.