Use secrets and fall back to random.SystemRandom for keys

Use secrets and fall back to random.SystemRandom for keys

The random module uses the Mersenne Twister pseudorandom number
generator and is not a cryptographically secure random number
generator[0]. The secrets[1] module is intended for generating
cryptographically strong random numbers, so recommend using that to
generate the secret key. It's new in Python 3, so if it's unavailable
fall back to using the ``os.urandom()`` backed implementation of random.

NOTE(stephenfin): Modified to include change to 'config.yaml'. Also
renamed reno to just stick with hyphens for filenames.

[0] https://docs.python.org/3/library/random.html
[1] https://docs.python.org/3/library/secrets.html

Signed-off-by: Jeremy Cline <jcline@redhat.com>
Signed-off-by: Stephen Finucane <stephen@that.guru>