Invalidating reports may fail because of log delete settings

[mattab]

How to reproduce original error:

• Enable log deleting in config.ini.php with small interval (i.e. 1 day)
• override those options via UI with bigger interval (i.e. 90 days)
• try to use invalidateArchivedReports() method with date between 1 and 90 days back from today - it will cause warning and reports for given date will not be invalidated even though they actually could be.

see #206

[mattab]

In 2d41d1e: Merge pull request #206 from clearcode/addedMissingOptionCheckToInvalidateArchivedReports

Switched from reading raw config values to reusing the helper method, fixes #4568