Re-enable flake8 check

This is the final fix to re-enable flake8/pep8 check in the gate.
E265 errors in doc/source/conf.py are also fixed.

Imports with "from xxxx import (xxxx)" style cannot pass "tox -e pep8"
though they pass "run_tests.sh -p". Newlines with backslash escape
is used in this commit to pass it.
The reason needs to be investigated later.

Change-Id: Ic0d765d404212d13f678b2a3aef7b9612bf5238d
Closes-Bug: #1347472