qa/tasks/cephfs: do no radom.sample(a_dict)

qa/tasks/cephfs: do no radom.sample(a_dict)

collect the keys instead of filtering a dict,
to address following failure:
```
2020-04-05T12:15:36.275 INFO:tasks.cephfs_test_runner:Traceback (most recent call last):
2020-04-05T12:15:36.275 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/cephfs/test_strays.py", line 29, in test_files_throttle
2020-04-05T12:15:36.275 INFO:tasks.cephfs_test_runner:    self._test_throttling(self.FILES_THROTTLE)
2020-04-05T12:15:36.276 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/cephfs/test_strays.py", line 96, in _test_throttling
2020-04-05T12:15:36.276 INFO:tasks.cephfs_test_runner:    return self._do_test_throttling(throttle_type)
2020-04-05T12:15:36.278 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/cephfs/test_strays.py", line 176, in _do_test_throttling
2020-04-05T12:15:36.278 INFO:tasks.cephfs_test_runner:    mds_max_purge_ops = int(self.fs.get_config("mds_max_purge_ops", 'mds'))
2020-04-05T12:15:36.279 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/cephfs/filesystem.py", line 285, in get_config
2020-04-05T12:15:36.279 INFO:tasks.cephfs_test_runner:    service_id = random.sample(filter(lambda i: self.mds_daemons[i].running(), self.mds_daemons), 1)[0]
2020-04-05T12:15:36.280 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/git.ceph.com_git_teuthology_py3/virtualenv/lib/python3.5/random.py", line 311, in sample
2020-04-05T12:15:36.280 INFO:tasks.cephfs_test_runner:    raise TypeError("Population must be a sequence or set.  For dicts, use list(d).")
2020-04-05T12:15:36.280 INFO:tasks.cephfs_test_runner:TypeError: Population must be a sequence or set.  For dicts, use list(d).
```

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 40d0356249193bf4aaa2d38923eef19caa63eb59)