(MODULES-1394) replace validate_db_connection type with custom type

This replaces the defined type validate_db_connection with a new
custom type called postgresql_conn_validator. It's functionality is
nearly a copy of its predecessor with the exception of the
create_db_first parameter which is taken care of with a collector in
service.pp. The old type is still intact but all docs have been removed
and a warning has been attached to it announcing its deprecation.