Add E2E test for channel default logic

[chizhg]

Problem
We don't have e2e test coverage for channel defaulter logic.

Exit Criteria
We should have an e2e test that:

1. Modify the default-ch-webhook configmap to change the default channel
2. Create a Channel with empty ChannelTemplate spec
3. Verify the Channel can work as expected, optionally check the created channel Kind is the same as set in the default-ch-webhook configmap

/assign

fyi @nachocano