dist_test: fix compatibility with newer 'isolate'
dist_test: fix compatibility with newer 'isolate'

The new 'isolate batcharchive' command puts the dumped hashes
directly in the top-level dictionary of 'dump.json' rather than
inside an 'items' dictionary. This fix makes the scripting compatible
with either version.

Change-Id: I6b1322b238202c0db9d403c8c21590eefa79de2e
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/6726
Tested-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>
Reviewed-by: Andrew Wang <7200923b3e219935cc07419228ef7ad73898b1ea@cloudera.com>

InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,EvilMcJerkface/kudu,andrwng/kudu,EvilMcJerkface/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu