Controller Config Version Override

[michaelquigley]

We have a new controller configuration (v: 4) coming out to support the limits changes. The production team wants to come up with a config file version that will use combined syntax to run on both v0.4.30 and the new v0.4.31.

To support this, I'm going to add support for setting a ZROK_CTRL_CONFIG_VERSION environment variable that will override the version of the config found in the file. This will make a file containing v: 3 work on a v: 4 controller, and yet will still break when v: 5 comes out.

The presence of this environment variable on certain controller versions and the absence on others will allow for mixed version configurations when the syntax is amenable to that working.