Use staged joins now that we don't need to test riak older than 1.2

Using staged joins, when building a 4 node cluster, is about a minute
faster than doing 4 sequential joins due to the elimination of redundant
handoff.
Only use staged joins for build_cluster
Merge pull request #357 from basho/adt-staged-join

Use staged joins now that we don't need to test riak older than 1.2