Don't run unnecessary node at every riak_test

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 in my laptop.

Save 7 minutes!