Fix HBase region splitting for tests.

It appears that HBase sometimes ignores an admin.splitRegion() RPC,
which made our region splitting fail. As a workaround, this patch adds
another retry loop such that the split/wait sequence is attempted
multiple times.

Change-Id: I9aa8ab87bba79ea11b79c50f15328b8be844924d
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4557
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: Alex Behm <alex.behm@cloudera.com>