[DS-2128] Fix 'ant fresh_install' breakage

1)  PostgreSQL driver returns "postgresql" rather than "postgres", which breaks path construction when initializing the database.  Invent a "DSpace DBMS keyword" which will have a value that we control, and use it here.

2)  Validation query was set before knowing whether it should be Oracle-style or nearly-everything-else-style.  Set no validation until we know which DBMS is in use.