Reverse commons-config automatic list parsing

Rexster calls TitanGraphConfiguration#convertConfiguration with what
Apache Commons Config calls a "hierarchical" configuration.  This
subset of Commons Config automatically turns comma-separated string
values into lists.  TitanGraphConfiguration needs to join these lists
back together as Strings for e.g. passing a list of seeds to a
Cassandra backend.  RexsterTitanServer does not use hierarchical
configurations and is not exposed to this automatic list parsing.