Added information on renaming a "single-node cluster"

While this might seem obvious, it's probably best not left to the
imagination.  Since the multi-node process includes a step that is
impossible in a single node setup (downing the node), I just made an
explicit section for single-node clusters.
Added path to ring_state_dir
Merge pull request #659 from basho/cv-rename-single-node

Added information on renaming a "single-node cluster"