Fix leaking of image BDMs

Patch Ia89d531be71c460f1f82fcfce34b270639a23061 introduce creating an
image block device for every instance, to comply with the new block
device format being introduced currently.

However, due to a mistake during rebase the needed code to delete those
once the instance was being deleted was broken (even though comments
suggest otherwise).

This patch now fixes this issue, by adding a proper flag to the
conductor call when getting BDMs in compute manager's _complete_deletion
method.

Fixes bug: #1206890

Change-Id: I6a9673758b0762f99f27860b8f88a16127da7399
Merge "Fix leaking of image BDMs"