Configuring Peribolos for Github org management

[munnerz]

We can use Peribolos for github org management, allowing us to manage who is a member of various organisations through GitHub using bots.

You can see an example of the config for this here: https://github.com/kubernetes/org/tree/master/config

@simonswine I know you've expressed a desire to manage a lot of this with Terraform instead - keen to hear your thoughts here.

This would be good at least to get setup for the cert-manager org, which is currently completely manually managed.

[inteon]

I started implementing this here: cert-manager/org#1