Improved workshop download reliability

[giovanni-grieco]

I've slightly modified the a3update.py to be more reliable. The problem is that steamcmd times out when downloading big mods.

So the main changes are that instead of launching steamcmd with the "+argument" I made it so that it launches using a .txt file. The txt file is generated by a3update.py it self and then when the workshop download has to be called, it launches steam with the specified script file. From what I've read online this system makes it more reliable.

2 more environment variables have been added:

• CHECK_MODS enables/disables the mods download and validation. If it's set to 0 it wont check for the preset.html and won't validate already downloaded mods. When CHECK_MODS is set to 0 it will still launch the server with all the previously downloaded mods.
• DOWNLOAD_ATTEMPTS specifies the amount steamcmd workshop download iterations. Basically, again, it's a workaround for the unreliable mess that steamcmd is.