added new/legacy splunk indexers to backups undeseried list

This change removed three retired systems from the "backups undesired" list: splunk-idx-4.cc.vt.edu,splunk-idx-5.cc.vt.edu, splunk-idx-6.cc.vt.edu. This request all adds the three "new legacy" indexiers to the list: splunk-idx-1.cc.vt.edu, splunk-idx-2.cc.vt.edu, splunk-idx-3.cc.vt.edu. I discussed some time ago with MPA that there was enough hardware redundancy to see us through until these systems are retired (schedule in the next couple of months).