Properly set system namespace when reconciling default brokercell

[ian-mi]

Fixes #1704

Proposed Changes

• Fix brokercell reconciliation so that it will be triggered by broker and trigger updates. Currently after creating or updating a broker the brokercell config will not be updated until it hits the resync period causing requests made to that broker to fail.

Release Note

- 🐛 Fix broker config reconcilation delay.

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-google-knative-gcp-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/brokercell/controller.go	48.7%	54.3%	5.6

[liu-cong]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ian-mi, liu-cong

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