Fix building of documentation

Now both the standard command "python setup.py build_sphinx" and
"tox -e docs" will build the documentation.

The changes:
* tox.ini: Fix docs section: Remove unneeded deps line, the general
  deps line is just fine. Enable the command, clean up before running
  sphinx and use the normal command for execution.
* pydocs.rst: This throw an error while building since
  troveclient.client.Dbaas does not exist, replace it.
* index.rst: Generate table of content. No need to include the files.

Change-Id: I2095ef73a76b0212e33776af925f5eaf1810be7a
Partial-Bug: #1315101