Remove most of the hard-coded Postgres connection defaults.

 * pyrseas/{dbaugment, yamltodb}.py: Remove localhost and port
   arguments to main() and eliminate call to parser.set_defaults.
 * pyrseas/dbtoyaml.py: Remove localhost and port arguments to main()
   and only pass schema in call to parser.set_defaults.
 * pyrseas/lib/dbconn.py (DbConnection.__init__): Disregard default
   values for host and port.
 * pyrseas/lib/dbutils.py (pg_connect): Disregard default value for
   port.
 * pyrseas/testutils.py (DbMigrateTestCase._db_params): Return a list
   of parameters without empty values and disregarding default values
   for host and port.  (DbMigrateTestCase.{run_pg_dump, create_yaml,
   create_yaml_dir, migrate_target, migrate_target_dir}): Exclude host
   option flag from initial args.