Switch over to oslosphinx

Having the sphinx theme in the same namespace package
as the production code has been causing issues with
devstack installations. The solution settled on was
to rename oslo.sphinx to oslosphinx because oslo.sphinx
is not a "production" library.

See the linked bug report for more background details.

Closes-Bug: #1277168
Change-Id: I419fe50d66fa0c1ac48deca2286ae7c4b922d72d