Triggers are allows to be created first.

There is a line in the control plane spec:

> Triggers MUST be associated with a Broker based on the `spec.broker` field on the Trigger;

which reads to me that triggers are not allowed to point to a non-existing broker, and they are allowed to do that.

Please clarify.

/cc @duglin @evankanderson

Start with the control plane spec passage quoted in the issue and trace how `Trigger`, `Broker`, and `spec.broker` are defined. The work is done when the specification clearly states whether a Trigger may reference a non-existing Broker and the relevant wording is clarified.

Written by the indexing model from the issue text.