qa/tasks/cephfs: annotate variable without comment

qa/tasks/cephfs: annotate variable without comment

to silence the warning like
```
flake8 run-test: commands[0] | flake8 --select=F,E9 --exclude=venv,.tox
./tasks/cephfs/test_full.py:6:5: F401 'typing.Optional' imported but unused
```

since we've migrated to python3.6 and up, there is no need to be
compatible with Python 2 anymore.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
(cherry picked from commit 2413da8c854bc46e79ca8d6e84f789f99747c203)

Conflicts:
	qa/tasks/cephfs/test_full.py [ commit 485841b255e1 ("qa: import
	  CommandFailedError from exceptions not run") not in pacific ]