Refactor DB cache management in runtests and runmigration

[yvaucher]

Gives most variables a default value making them changable with var=${var:=default}

Extract db restore and dump in a separate db.sh file.

Change (default) value for cachedb directory to make it easier to set a volume on it.

Add || true in runmigration when doing ls -v to ignore the error status and set an empty CACHED_DUMP variable.

Improve slightly the messages.