vrrp: add a systemd credential drop-in sample for auth_hmac keys

Document feeding the auth_hmac key to keepalived as a systemd credential
so the secret stays out of keepalived.conf and off disk at rest. The
sample drop-in seals the key with LoadCredentialEncrypted, and the config
locates it with file:${_ENV CREDENTIALS_DIRECTORY}/<name>, relying on
${_ENV} parameter substitution to expand with the trailing path.

Signed-off-by: Alexandre Cassen <acassen@gmail.com>