Update openstack-doc-tools to 0.6

0.6 is the latest release and a hard requirement for gating
the documentation projects.

Change-Id: I9a3e7f397c3e41f0c1e1d3f500c3eb6ff6b447df
Sort global-requirements

Change-Id: Ibef54e6f3aea36515ef354d097108759713c7b32
Merge "Sort global-requirements"
Merge "Update openstack-doc-tools to 0.6"