INT-3074: JDBC: Make generateSql as UP-TO-DATE

* remove `overwrite: 'true'` from `generateSql` task
* add `cleanSql` task

Now, if we change some schema template (.vpp or .properties),
there is need to clean .sql files

JIRA: https://jira.springsource.org/browse/INT-3074

**Cherry-picked to 2.2.x**