Fix typos and code style check

In neutron/api/extensions.py, there are some typos, no needed blank
lines and docstrings can be improved (according to HACKING.rst).

Closes-Bug: #1213541

Change-Id: I01087d133e1b2a7e69f0bcc2ae359a3a2042b1dc
Merge "Fix typos and code style check"