The behaviour of PY3's standard lib 'argparse'
differs from PY2's.
```
cinder extend 2
usage: cinder extend <volume> <new_size>
error: too few arguments
Try 'cinder help extend' for more information.
cinder extend 3
usage: cinder extend <volume> <new_size>
error: the following arguments are required: <new_size>
Try 'cinder help extend' for more information.
```
This could lead to the partly failure of functional testcase,
fix it.
Change-Id: I59e9ae149af0b4294b09a94a52a4bc86a1d90f2b
Closes-Bug: #1697428