Issue 50960 - [RFE] Advance options in RHDS Disk Monitoring Framework
    
    Description: Add an attributes to cn=config
    
    nsslapd-disk-monitoring-readonly-on-threshold - it accepts 'on/off'
    values and changes the original behavior so when we reach the threshold,
    we set the read-only mode (and then we do other actions - like disable
    non-critical logging, etc.). When we reach half of the threshold, we start
    the graceful shutdown as it was before.
    
    Change the code so the instance doesn't start up if the disk space
    is still below half of the threshold.
    
    Add basic tests to disk monitoring test suite.
    
    https://pagure.io/389-ds-base/issue/50960
    
    Reviewed by: firstyear, tbordaz (Thanks a lot!!)