If nova-manage cell_v2 create_cell returns 2, it's because the cell already
exists and all instances are correctly mapped to the cell. The cell_v2
map_instances doesn't need to run.
Change-Id: I38002a18dff4dbd44bd3df9e1969c9d50ab12705
Related-Bug: 1729661
(cherry picked from commit 0f24bb8bd4)