Fix runmigration cached using DB load and create

[yvaucher]

Allow to set restore a cached DB and create a new one

If both LOAD_DB_CACHE (with a ceil version) and CREATE_DB_CACHE were set, in case the directory containing cached DB is empty the CACHED_DUMP variable was emptied and the script was unable to create a new dump.

It also prevent to rename the new cache dump with the previous dump name.