BrokerCell creation controller

[grantr]

Problem
The first time a Broker is created, we should create a BrokerCell to service it. This can be a separate controller from the normal Broker and BrokerCell controller for simplicity.

If this BrokerCell is modified, the controller shouldn't attempt to reconcile away the differences. If the BrokerCell is deleted, the controller should create a new one.

This controller is optional but recommended since it reduces the footprint of eventing when no eventing objects have been created.

Time Estimate (optional):
2

Additional context (optional)
If we later add support for multiple BrokerCells (see #866) this controller should create the BrokerCell a Broker is assigned to if it doesn't exist.

[grantr]

This is now P1 based on outcome of #1077