Add tool to document cli tools

Call "os_doc_tools/commands.py" nova to generate a file
section_cli_nova_commands.xml with the command line options.

Allows generation of all supported clients with --all command.
The tool gets installed as: openstack-auto-commands

blueprint os-user-doc
Change-Id: Ie4b200818bd585d3fce0b27f0639ad03069b3b7b