dist_test: enable sharding of a few more tests
dist_test: enable sharding of a few more tests

Adds sharding for rowset_tree-test, tablet_copy-itest, and
delete_tablet-itest which are the longest-running non-sharded tests in
recent builds such as [1]

[1] http://dist-test.cloudera.org/job?job_id=jenkins-slave.1510801703.18407

Change-Id: Iff1e0ac39f1834e8ac22283e1dc0c336d328ae35
Reviewed-on: http://gerrit.cloudera.org:8080/8565
Reviewed-by: Dan Burkert <4ef91e292a55314d2653d35ff669a6bd939ce515@apache.org>
Tested-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

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