Add variable privileged user for DB imports

[mkayontour]

Right now all imports are done using the username/password combination which is necessary for the connection. But those aren't always privileged enough to import schemas.

Check if privileged user is given and then use those at the import instead.

[mocdaniel]

Which parts of the Icinga stack would be affected by this? For modules, we can now pass different credentials to the icingaweb2_modules.<module>.database.username/password part than the ones configured in icingaweb2_resources.<moduledatabase>.username/password, so the problem is already solved there.

This leaves two components if I'm not mistaken:

• icingaweb2 itself (configured via icingaweb2_db at the moment)
• IcingaDB's database, which gets bootstrapped by the IcingaDB daemon.

So we would need to add this way of importing schemas only for Icingaweb2's config database, then? Would it make sense to expand the import schema feature used for modules so that it can handle this use-case, too?