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 ;))

UPD: (while jira is down, extra comments):

1) It was tempting to remove documentation for this, as we don't have "reset" there. But may be it is fine to keep it
2) also, we discussed long time ago that these operations: reset, delete all (datasets, etc.), etc. - should not be handled by running cdap instance, but rather done thru the offline tool, similar to upgrade. Not right now - as we are undergoing other major changes