Fix multiple TFA instances

[mcmikemn]

If traefik-forward-auth is installed on multiple servers hosting apps in the same domain, all the TFAs default to the same cookie name ("_forward_auth") but each has a different secret. This setup causes the different servers in the same domain to interfere with each other's cookies.

One solution is to give each TFA instance the same secret.

The solution we're going to use is to keep each TFA with its own secret but each will have a unique cookie name.