Remove the 'openstack' subdirectory of nova/api

Originally the nova/api directory was supposed to contain several APIs for nova. The only ones that ever existed were 'ec2' and 'openstack'. Now that ec2 is gone, and we have no plans to add others, we should remove the 'openstack' directory, promoting it contents up to the 'api' parent directory. The 'ec2' directory can still remain for its current tombstone marker function. This is obviously low priority, but it would make life a little easier for those of us working in nova/api/openstack