qa/tasks/cephfs: cast mds_recall_warning_decay_rate to float

qa/tasks/cephfs: cast mds_recall_warning_decay_rate to float

this change should address the failure of
```
2020-04-05T15:14:23.088 INFO:tasks.cephfs_test_runner:Traceback (most recent call last):
2020-04-05T15:14:23.088 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/cephfs/test_client_limits.py", line 110, in test_client_pin_mincaps
2020-04-05T15:14:23.089 INFO:tasks.cephfs_test_runner:    self._test_client_pin(True, 200)
2020-04-05T15:14:23.089 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/cephfs/test_client_limits.py", line 71, in _test_client_pin
2020-04-05T15:14:23.090 INFO:tasks.cephfs_test_runner:    self.wait_for_health("MDS_CLIENT_RECALL", mds_recall_warning_decay_rate*2)
2020-04-05T15:14:23.091 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/ceph_test_case.py", line 152, in wait_for_health
2020-04-05T15:14:23.091 INFO:tasks.cephfs_test_runner:    self.wait_until_true(seen_health_warning, timeout)
2020-04-05T15:14:23.092 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/ceph_test_case.py", line 193, in wait_until_true
2020-04-05T15:14:23.093 INFO:tasks.cephfs_test_runner:    if elapsed >= timeout:
2020-04-05T15:14:23.093 INFO:tasks.cephfs_test_runner:TypeError: unorderable types: int() >= str()
```

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