qa/tasks/ceph_manager.py: do not return a filter

qa/tasks/ceph_manager.py: do not return a filter

as the caller might want to `len(manager.get_osd_status()['raw'])`, and
`len()` does not accept a `filter` object.

also, the filtered osd statuses are printed out using `self.log()`, so
we should materialize the `filter` object before sending it to logging
facility. otherwise we will have something like:
```
2020-04-08T02:58:37.001 INFO:tasks.ceph.ceph_manager.ceph:<filter object at 0x7f5a080e1518>
```
in the logging message.

Signed-off-by: Kefu Chai <kchai@redhat.com>