Import markdown-docbook.sh from config

Import file from openstack-infra/config repository with filename
modules/jenkins/files/slave_scripts/markdown-docbook.sh

The file will be used for converting markdown to docbook.

Add this separately so that we can track changes to it more
easily.

Change-Id: I8d3a13d4b1126fa8845e2b5d5a4a9db6b23fbc64
Merge "Import markdown-docbook.sh from config"