upgrade to Riak 2.0

This is a start of a series of updating Riak 2.0. To update your riak_test, update your Riak harness to the latest 2.0.x. Currently, there are no 2.0 specific thing so even after this commit, CS might work with Riak 1.4 and 2.0. My example .riak_test.config is as follows:

```erlang
{rt_cs_dev, [
  {rt_project, "riak_cs"},
     {rt_deps, [
                "/home/kuenishi/cs-2.0/riak_cs/deps"
               ]},
     {rt_retry_delay, 500},
     {rt_harness, rt_cs_dev},
     {build_paths, [{root,              "/home/kuenishi/rt/riak_ee"},
                    {current,           "/home/kuenishi/rt/riak_ee/current"},
                    {ee_root,           "/home/kuenishi/rt/riak_ee"},
                    {ee_current,        "/home/kuenishi/rt/riak_ee/current"},
                    {ee_previous,       "/home/kuenishi/rt/riak_ee_onefour/current"},
                    {cs_root,           "/home/kuenishi/rt/riak_cs"},
                    {cs_current,        "/home/kuenishi/rt/riak_cs/current"},
                    {cs_previous,       "/home/kuenishi/rt/riak_cs/1.5"},
                    {stanchion_root,    "/home/kuenishi/rt/stanchion"},
                    {stanchion_current, "/home/kuenishi/rt/stanchion/current"},
                    {stanchion_previous,       "/home/kuenishi/rt/stanchion/1.5"}
                   ]},
     {test_paths, ["/home/kuenishi/cs-2.0/riak_cs/riak_test/ebin"]},
     {src_paths, [{cs_src_root, "/home/kuenishi/cs-2.0/riak_cs"}]},
     {lager_level, debug},
     {build_type, oss},
     {flavor, basic},
     {backend, {multi_backend, bitcask}}
]}.
```

I also recommend preserving your old CS 1.5 test firm with name like `rt_cs_dev_old`. As this is still WIP state, I'll add several upgrading tests.

CC: #962
Ah, I moved around here and this PR description. The description says `{cs_previous,       "/home/kuenishi/rt/riak_cs/1.5"},`.