Refactor compute tests to not use _objectify()

During the objects conversion of nova-compute, we ended up with a lot of
places in the tests where we would take a dict or sqlalchemy object
and "objectify" it to pass it to a method that had been converted to
expect an obejct. That work is done now, but we have a LOT of cruft in
that code that needs to be cleaned up. This patch does that, and removes
the _objectify() helper.

Change-Id: Iee3e9ea619417178618f8f6c192097444899dd26
Fix instance_extra backref

This fixes cruft from the original NUMA patches that placed a backref on
the Instance model for numa_topology. This changes it to the generic
'instance_extra'. Since this was not actually used, nothing breaks; the
extra backref will be used in subsequent patches.

Related to blueprint flavor-from-sysmeta-to-blob

Change-Id: Ifa69e4704ea3f60b4aced3504bf9c599e3868616
Merge "Refactor compute tests to not use _objectify()"
Merge "Fix instance_extra backref"