Add unit test to update server metadata

Fixes bug: 1088878
Partially implements blueprint nova-v2-api-audit

The bug description points to 4 situations to be tested, although those
situations are already tested on create metadata tests which use the same
method as update_all to operate over metadata (_update_instance_metadata) so no
more tests are needed.

Change-Id: Ifae3c69c0e00c361ae36f5a8dede835c96568d11
Merge "Add unit test to update server metadata"