configure cron expression to merge remote indexes tru the UI

git-svn-id: https://svn.apache.org/repos/asf/archiva/trunk@1551471 13f79535-47bb-0310-9956-ffa450edef68
refactoring to ease cron scheduling

git-svn-id: https://svn.apache.org/repos/asf/archiva/trunk@1551472 13f79535-47bb-0310-9956-ffa450edef68
[MRM-1746] build merged index for groups with a cron schedule

git-svn-id: https://svn.apache.org/repos/asf/archiva/trunk@1551473 13f79535-47bb-0310-9956-ffa450edef68