Update openstack-doc-tools to at least 0.15

Version 0.15 is needed to build the HA guide in openstack-manuals
and to build localized security-guide.

0.15 contains the following changes over 0.13:

0.15
----

* ``openstack-doc-test``: Output information about tested patch,
  special case entity files for book building. Remove special handling
  for high-availability-guide, it is not using asciidoc anymore.
* New script in cleanup/retf for spell checking using the RETF rules.
  patch.
* Fix entity handling in ``openstack-generate-docbook``.

0.14
----

* ``openstack-auto-commands``: Improved screen generation and swift
  subcommand xml output.
* ``openstack-doc-test``: Warn about non-breaking space, enhance
  -v output, special case building of localized high-availability
  guide, fix for building changed identity-api repository.
* New command ``openstack-jsoncheck`` to check for niceness of JSON
  files and reformat them.
* ``openstack-autohelp``: Update the default parameters. The tables
  are generated in the doc/common/tables/ dir by default, and the git
  repository for the project being worked on is looked at in a sources/
  dir by default.

Change-Id: Ib6b0360f4443c80579f2aeba55642c0fbb3da3f6
Merge "Update openstack-doc-tools to at least 0.15"