Doc: oslo.sphinx -> oslosphinx

The sphinx theme has been renamed as seen in I87ca3e6c10. Side effect:
that will bump the theme to version 2.0.

There is still an occurence of 'oslo.sphinx' in the gerritbot:
  modules/gerritbot/files/gerritbot_channel_config.yaml

No idea what it is for so I left it untouched.

Change-Id: Id63db363470c85c8a151be6643f6810e653fb8de
Merge "Doc: oslo.sphinx -> oslosphinx"