Add legacy flag to get_instance_bdms

In change If30afdb59d4c4268b97d3d10270df2cc729a0c4c for flexibility
reasons, it was decided to not change the method API.get_instance_bdms
to return legacy format but to change all the places it was called.
Sadly few of them were missed.

This patch adds a legacy flag so that all use-cases still do what is
expected unless explicitly changed to do otherwise.

fixes bug: 1191913
blueprint: improve-block-device-handling

Change-Id: Iaebbb851e4a2a9cbee13fa271bf15d789db23f24
Merge "Add legacy flag to get_instance_bdms"