Test that ensemble delete functions in yokozuna

This tests that basho/yokozuna#404 functions as planned, as well as acts as a placeholder for more ensemble yz tests. This test lives in r_t, apart from other yokozuna tests, to use the `ensemble_util` testing module. More tests are to be added.
rt:pbc_write/5 doesn't seem to exist. It's causing the following failure. `"text/plain"` probably needs to be removed.
14:49:13.162 [info] Writing 1023 keys
14:49:13.163 [warning] yz_ensemble failed: {undef,[{rt,pbc_write,[<0.2874.0>,{<<"strong">>,<<"test">>},<<0,0,0,0,0,0,0,1>>,<<0,0,0,0,0,0,0,1>>,"text/plain"],[]},{yz_ensemble,'-verify_ensemble_delete_support/3-lc$^1/1-1-',3,[{file,"tests/yz_ensemble.erl"},{line,53}]},{yz_ensemble,verify_ensemble_delete_support,3,[{file,"tests/yz_ensemble.erl"},{line,53}]},{yz_ensemble,confirm,0,[{file,"tests/yz_ensemble.erl"},{line,38}]},{riak_test_runner,return_to_exit,3,[{file,"src/riak_test_runner.erl"},{line,159}]}]}
14:49:13.163 [error] Error in process <0.208.0> on node 'riak_test@127.0.0.1' with exit value: {undef,[{rt,pbc_write,[<0.2874.0>,{<<6 bytes>>,<<4 bytes>>},<<8 bytes>>,<<8 bytes>>,"text/plain"],[]},{yz_ensemble,'-verify_ensemble_delete_support/3-lc$^1/1-1-',3,[{file,"tests/yz_ensemble.erl"},{line,53}]},{yz_ensemble,verify_ensemble_delete_support,3,[{file,"tests/yz_en...


14:49:13.163 [error]
================ yz_ensemble failure stack trace =====================
{undef,[{rt,pbc_write,
            [<0.2874.0>,
             {<<"strong">>,<<"test">>},
             <<0,0,0,0,0,0,0,1>>,
             <<0,0,0,0,0,0,0,1>>,
             "text/plain"],
            []},
        {yz_ensemble,'-verify_ensemble_delete_support/3-lc$^1/1-1-',3,
                     [{file,"tests/yz_ensemble.erl"},{line,53}]},
        {yz_ensemble,verify_ensemble_delete_support,3,
                     [{file,"tests/yz_ensemble.erl"},{line,53}]},
        {yz_ensemble,confirm,0,[{file,"tests/yz_ensemble.erl"},{line,38}]},
        {riak_test_runner,return_to_exit,3,
                          [{file,"src/riak_test_runner.erl"},{line,159}]}]}
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