It's a good idea to be able to specify SRS secrets.

• If you're running a cluster, you'll need to have the same secret on each node to be able to accept DSNs consistently
• Having a predictable secret baked into the image is bad for security. The secret is generated in the deb's postinst so you'll have a known secret for each build. That can be leveraged to create an open relay

Further work: I'll probably get another PR going to generate a secret and store it in state