Update openstack-doc-tools to 0.20

We need the new version to build the heat documentation in
openstack-manuals.

List of changes for openstack-doc-tools:

0.20
----

* ``openstack-doc-test``: Check for a ``\n`` in the last line of a file.
* ``openstack-dn2osdbk``: Properly handle internal references.

0.19
----

* ``openstack-doc-test``: Optimize translation imports, improve output
  messages.
* ``autohelp.py``: Improve sanitizer, better support for i18n in
  projects, allow setting of title name for tables.
* ``autohelp-wrapper``: Smarter handling of the manuals repo and
* environment
  setup, add support for the ``create`` subcommand.
* ``autohelp-wrapper``: Add support for offline/fast operation.
* ``autohelp-wrapper``: Add a module blacklisting mechanism.
* ``diff_branches.py``: Updated output format.
* Provide a ``hotref`` extension for sphinx, to automate the creation of
  references to the HOT resources documentation.
* ``openstack-auto-commands``: Handle python-openstackclient, handle
  python-glanceclient and python-cinderclient v2 commands.

Change-Id: I0a14023120e25af9b74eb62529f4090da0f56497
Merge "Update openstack-doc-tools to 0.20"