delete all datasets to delete only user datasets

https://issues.cask.co/browse/CDAP-333 (jira is down at the moment, sorry).

Note the API change: now delete all datasets is DELETE request to:
```
/unrecoverable/data/datasets
```
This makes it possible to delete only user datasets without wiping out all the data and also makes it consistent with:
```
/unrecoverable/reset
```

Will put more comments in jira's discussion (once it is up ;))