Create default BrokerCell on installation

[grantr]

Problem
Currently we create the default BrokerCell only when the first Broker is created. This makes it impossible to alter the default BrokerCell's parameters before creating a Broker. See #1671 for a case where this would be useful.

The original idea of creating the default BrokerCell on demand was to minimize idle resource requirements. We can still accomplish this by scaling all deployments down to zero when no brokers exist (or, after #866, when no brokers are assigned to the BrokerCell).

Exit Criteria
A BrokerCell exists and can be modified before the first Broker is created.

Time Estimate (optional):
How many developer-days do you think this may take to resolve?

Additional context (optional)
If the BrokerCell is created by an operator during installation, it must use an "EnsureExists" strategy rather than attempting to reconcile changes.

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