When target_cell was originally written, the intent was to yield the
context that should be used. It currently mutates the input context,
which means you don't have to actually use the yielded one, because
we have a lot of stuff that would break otherwise. This fixes all the
current uses of it to be proper, and adjusts tests accordingly. This
is separate from changing target_cell's behavior to not mutate the
input context specifically to isolate the mechanical changes from ones
that actually need different behavior.
In addition to code that was already using target_cell() but not
depending on the yielded context, the _create_block_device_mapping()
method in conductor/manager.py was still depending on the shared
context switching to target the BDM objects on create. Since these
were prepared with the context prior to having determined where the
instanace was going to end up, we need to explicitly target the
object context on create (like the rest of the boot workflow does
for other objects in schedule_and_build_instances()).
Related to blueprint cells-aware-api
Change-Id: I35206e665f2c81531a2269dd66f8c5c0df834245
bb7c6ebb56