Issue 51029 - Add db_home_dir defaults.inf
Description: Latest lib389 from master requires a new key
'db_home_dir' in defaults.inf that is missing in the older
builds of 389-ds-base.
Add the key to defaults.inf so we can run the tests.
https://pagure.io/389-ds-base/issue/51029
Reviewed by: mreynolds (Thanks!)