Operator should not allow multiple Contours to exist with same Gateway API Controller string

[sunjayBhatia]

Please describe the problem you have
Once #386 and #389 are complete, users should be able to configure a controller string per-Contour instance. The Operator should no longer require a hard coded controller name.

The Operator should in turn not allow users to create multiple Contour instances with the same controller string, otherwise these instances would fight over the status etc. of their relevant Gateway/GatewayClass (also for Contour it doesn't really make sense to have two instances reconciling the same thing)