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>
Merge "Fix Flake8 style warnings in horizon/"