switch back to ibm driver (pdo_ibm->ibm_db2)

* db2pdo seems to be not very well supported event though though
  the API of PDO is somhow nicer

Change-Id: Ib198630e3a0b558407891b96aaa4f01481200c35
rewrite db2 test as php unit test

Change-Id: I4bcd2c1950fc8facd56f4ca24310fdfab52102ca