Act 2107

This pull request fixes ACT-2107, where the engine database update from 5.14 to 5.15 will fail on an Oracle database schema that has been through an export/import cycle. There are two commits, related:

* First, DbSqlSession.executeSchemaResource() has been updated, so that it is possible to specify that a sql-command may fail without failing the upgrade.
* Next, activiti.oracle.upgradestep.514.to.515.engine.sql has been upgraded with the required "drop index" statement, using the above feature to avoid failures on schemas where export/import does not fail.