Provide a script to gerenate options changes

diff_branches.py generates a listing of the configuration options
changes that occured between 2 openstack releases.

This involves a few changes in other tools:
- the 'dump' subcommand for autohelp.py generates the serialized dict of
  options
- add a special case for the 'bindir' option to avoid getting different
  default values in different virtual environments
- the autohelp-wrapper -e switch builds the needed venv without
  running autohelp.py commands

Change-Id: I80da172b91b8d2f0a15f89f4c812864da2fea471
Merge "Provide a script to gerenate options changes"