Provide an ability to select beta branches

[milkywayfarer]

In entry.sh script, make it possible to specify the beta version, setting the default to public. Something like STEAMAPPBETA variable, so that the result on line 44 looks like:

+app_update "${STEAMAPPID}" "${STEAMAPPBETA}" "${VALIDATE}"\

The problem I encountered: trying to fix the server version, I just overrode the STEAMAPPID variable to 730 -beta 1.41.2.8. But then, even if I removed this statement, the tag stayed the same locally and steamcmd didn't update the game.

I then had to manually get to each server and set the STEAMAPPID to -beta public to fix that.

I don't think the depo would change that much, and hardcoding public as default value seems okay.

[joedwards32]

Thanks for the implementation details. I'll look at adding this functionality soon.