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/"