Add tool to merge passwords during release upgrade

Typical usage:
   cp /etc/kolla/passwords.yml /etc/kolla/passwords.yml.old
   kolla-genpwd
   kolla-mergepwd --old /etc/kolla/passwords.yml.old \
                  --new /etc/kolla/passwords.yml     \
                  --final /etc/kolla/passwords.yml

Change-Id: Ibbc598909e28e096145841aec929d0cfcf8f7cab
Implements: blueprint kolla-merge-passwords