Oracle: TableManipulation.tableExists fails, when user has extended permissions

When an Oracle user has e.g. permissions to visit other schemas, the metaData.getTables returns not only the table matadata of the schema of the current user. So if a table exists in another user it returns true.

Solution: Since we anyway do not support explicit definition of schema name in configuration, we can use in case of oracle the current user name as schema pattern for getTables(..)
Other databases has different structures, e.g. in mysql the schema param seems to be ignored. To avoid problems the chance should be limited to oracle at the moment.