Currently nova-manage map_instances uses a marker set-up by which repeated
runs of the command will start from where the last run finished. Even
deleting the cell with the instance_mappings will not remove the marker
since the marker mapping has a NULL cell_mapping field. There needs to be
a way to reset this marker so that the user can run map_instances from the
beginning instead of the map_instances command saying "all instances are
already mapped" as is the current behavior.
Change-Id: Ic9a0bda9314cc1caed993db101bf6f874c0a0ae8
Closes-Bug: #1745358