Flake8: Fix F841

Enable F841 and fix all issues.

Flake F841 is:
F841 local variable 'XXX' is assigned to but never used

Also fix in changed files some occurences of:
E128 continuation line under-indented for visual indent
H404  multi line docstring should start without a leading new line
H405  multi line docstring summary not separated with an empty line

Change-Id: Ifb08a82d05ac15c8e90f214a96e73d82ff850018
Partial-Bug: #1333290
Merge "Flake8: Fix F841"