Make instance show and index use objects

This makes compute_api return instance objects for its
get() and get_all() methods. The necessary changes to
nova-api's compute/servers are included, which are
pretty minor. Most of the change in this patch is making
the tests do the right thing after the switch to objects.

Also, make a mostly-unrelated typo fix in
obj_to_primitive() which was missed in a previous review.

Related to blueprint compute-api-objects

Change-Id: I08dddd311f061e3b39df953b1c8f21f778603770
Merge "Make instance show and index use objects"