MH-9393 -- Fixed cleanup operation

git-svn-id: http://opencast.jira.com/svn/MH/trunk@13743 e6edf6fb-f266-4316-afb4-e53d95876a76
MH-9388 -- made artifact availability check in download distribution service optional

git-svn-id: http://opencast.jira.com/svn/MH/trunk@13744 e6edf6fb-f266-4316-afb4-e53d95876a76