Require openstack-doc-tools >= 0.18.1

For gating the manuals, the new version is needed.

Significant changes in 0.18 are:
* ``openstack-doc-test``: Don't always build the HOT guide, add new
  option --check-links to check for valid URLs.
* ``openstack-dn2osdbk``: Allow single files as source.
* Imported and improved ``doc-tools-check-languages`` (recently known
  as ``tools/test-languages.sh`` in the documentation repositories).
* Added a virtual build and test environment based on Vagrant.

0.18.1 is a bug fix release that fixes doc-tools-check-languages.

Change-Id: I15fc3e689ffd90be4300d0bc423a96b73c823972
Merge "Require openstack-doc-tools >= 0.18.1"