We have some common code used by several docs scripts. Migrate that
into the build-only shared code repository.
* Move lib/ansible/utils/_build_helpers.py to the directory for common
build code
* Migrate docs/bin/dump_config.py to a build-ansible subcommand
* Migrate dump_keywords to the build-ansible framework
* Make the script more maintainable by using functions and good
variable names
* Port to Python3 idioms
* Fix bug so that private attributes will be undocumented
* Move generate_man to a build-ansible subcommand
* Port plugin_formatter to a build-ansible subcommand
* Rework command_plugins so that docs scripts can target Python-3.4+ and
releng-only subcommands can use more recent versions of Python.
The architecture is now that command_plugins/* need to be importable
on Python-3.4. The init_parsers() method needs to run on Python-3.4.
But the main() method can utilize features of more recent Python as
long as it fits within those parameters.
* Update docs build requirements
Port the plugin_formatter to build-ansible framework
