New environment variable TF2_BEHAVIOR

[bjourne]

This environment variable is required for TPUs to work through ssh. It was added today and it took me a while to figure out why my TPU models suddenly stopped working. :)

In launch_ssh_cloudflared.py I see that you have a "whitelist" with variables to export. May I suggest a "blacklist" approach instead, so that you export all environment variables found except for a list of ephemereal ones? I think that would be more future proof. The blacklisted ones would be LS_COLORS, SSH_CONNECTION, LESSCLOSE, USER, PWD, HOME, SSH_CLIENT, SSH_TTY, MAIL, TERM, SHELL, SHLVL, LOGNAME, PATH, LESSOPEN, and _ (underscore).