Rename package from oslo.sphinx to oslosphinx

Having the sphinx theme in the same namespace package
as the production code has been causing issues with
devstack installations. This change renames the package
to move it out of the namespace. Another change to the
openstack-infra/config repository will rename the
git repository and associated jobs.

Partial-bug: #1277168

Change-Id: I7788a9d6b5984fdfcc4678f2182104d2eb8a2be0