Allow A/B testing settings to be set via environment variable

[miketaylr]

This way we wouldn't need to deploy every time we want to tweak the branches, we would just restart uwsgi.

cc @karlcow @johngian

[karlcow]

so the goal here is to have to avoid to PR and redeploy the codebase for something which is essentially a configuration choice.

webcompat.com/config/__init__.py

Lines 203 to 212 in 6b5f252

	# AB testing config
	AB_EXPERIMENTS = {
	'exp': {
	'variations': {
	'form-v1': (0, 100),
	'form-v2': (100, 100)
	},
	'max-age': None
	}
	}

So we could have an environment variable, which can be read when we restart uwsgi.