Add generate_passwords.py to generate passwords

As with all tools, this is a first pass at the generation. Perhaps we
even want to move this into kolla/kolla/cmd and be generated with tox
itself in the future.

This tool, when run, will only populate empty fields that have no
values meaning that it is safe to run repeatedly on the same file.

Of note, there is no way to preserve comments in the file after it has
been processed by the yaml parser in python. Comments and sections
will remain in the passwords.yml template for additional documentation
if the user wishes to populate the file themselves.

Use SystemRandom and clean up the docs a bit to not use pronouns.

Co-Authored-By: Steven Dake <stdake@cisco.com>

Closes-Bug: #1559266
Change-Id: I2932d592df8871f1b7811059206d0b4d0553a687