Fix Flake8 style warnings in horizon/

Warnings H904, H307 and H405 are new or considerably changed, and will
be fixed in a separate patch.

Partial-bug: #1347472

Change-Id: Id9325a40d443f77242d3634002789501afbfc35a
Co-Authored-By: Juan M. Olle <juan.m.olle@intel.com>
Fix Flake8 style warnings in openstack_dashboard/

Warnings H904, H307 and H405 are new or considerably changed, and will
be fixed in a separate patch.

Closes-bug: #1349820
Partial-bug: #1347472

Change-Id: I4fd28990dacf16f03a4eaa6074ef59c37f1a2c14
Merge "Fix Flake8 style warnings in openstack_dashboard/"
Merge "Fix Flake8 style warnings in horizon/"
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