system: do not drop DB when cleaning

[tiborsimko]

The clean-instance.sh helper script currently destroys the DB, which is not good in DBoD usage scenarios. It should only empty the DB, not destroy it.

Let's rather have a clean-distinction strategy going like:

• provision X - install whatever is necessary for a service X to run on the given platform;
• create X - create instance or any necessary runtime services (COD3 instance, DB database, ES indexes, whatever)
• start X - start instance or service
• populate instance - upload records to the instance
• clean instance - "unpopulate" records from the instance, i.e. empty site content and its tables, but keep them otherwise ready; the site should still show, but be empty
• destroy X - delete COD3 instance and/or any necessary DB, ES resources

[ioannistsanaktsidis]

@hjhsalo do you want to take care of this one ?

[tiborsimko]

@ioannistsanaktsidis Yes, we'll take care of this on our end... I'll add a few more system tickets of this kind.