Fix doc gate job false success

pbr config was missing from setup.cfg. With adding "warnerrors = true"
under '[pbr]' in setup.cfg, the Sphinx build will fail in case of
warnings and errors during the build process.

This patch also fixes the currently existing problems in the developer
documentation of Ceilometer. The unused todo extension was also removed.

Fixes-Bug: #1317648
Fixes-Bug: #1321193
Fixes-Bug: #1272845

Change-Id: If1a9d7aca5c1d350568a83fb49bb8ea3defefdca
Merge "Fix doc gate job false success"