Not all riak_test at Riak CS have to run at four nodes, esp. CS node
No.2,3,4 are rarely used. Also Riak node dev2,dev3,dev4 participates
into the cluster. The time to setup unused nodes are getting longer
as number of tests increase.
This commit changes node number of most tests from 4 to 1, which
improved the time to run all tests from 22min => 15min.
changing node numbers because external client tests are rather
compliated. Also, lists_objects_v2 uses CS-specific cover operation
of Riak and therefore we'd better try beat it rather than saving
times.
Merge pull request #762 from basho/refactor/fewer-node-at-rt

Don't run unnecessary node at every riak_test