ENG-4178: Fix the schema to allow 0 or 1 snapshot elements within
partition-detection element.  Fix CatalogUtil to provide appropriate
default when not specified.  Add parsing and catalog content test
cases.
ENG-4122: Detect the case of a node attempting to rejoin to itself
earlier and print a sane error message.
Merge pull request #327 from VoltDB/eng4178

ENG-4178: Fix the schema to allow 0 or 1 snapshot elements within