Make sphinxbuild fail on formatting errors

This change will cause docs build to fail if rst is badly formatted.

Based on nova-specs change: I20e31fb25717388c22c54d5109afc4a56f9ab28d
Drop site packages and rename build_specs to docs

This commit drops site-packages from the tox.ini because it isn't
needed. It also renames the build_specs tox job to docs to be
consistent with the other projects.

Based on nova-specs changes:
Ic5cb48951b1031a682adf2c37ee8fdc8615c505d
and
I20ff3c1df99fb50fbe16361024a44b04c03f0a67
Allow running tox without any arguments

This commit adds an envlist to the tox.ini to enable running tox
without any arguments. This is based on nova-specs change:
I3137ce5ae5631bf5bcf0455a17828369dea6b9fd