Add handling of DISABLE_COLLECTORS env var and test

In order to reduce the chance of us exceeding our GA quotas, we needed a way to disable collectors but still know that they would have run.
Merge pull request #72 from alphagov/allow-disabling-of-collectors

Add handling of DISABLE_COLLECTORS env var and test
Add missing entrypoint arg
Merge pull request #75 from alphagov/fix-missing-entrypoint

Add missing entrypoint arg