Remove the OpenStack logo from our developer docs

oslophinx inserts the official OpenStack logo, which we are not allowed
to use. This patch just places a solum logo with the same file name that
the tweek.css expects.

Fixes-bug: #1295486
Change-Id: Icdf923a9ac8021e8d609fdc5d898d939cfed39be
Merge "Remove the OpenStack logo from our developer docs"