Avoid manipulating users VirtualService and Gateway

[aslakknutsen]

What can be improved

Given that we only want to support host based routing, aka require to use the exposed hostname to access the route, we should be able to get away with not modifying any of the user resources.

We already create a VirtualService bound to the exposed host name to add the header property, and we should be able to have multiple gateways with different hosts, we might get away with not manipulating the users VirtualService, but rather both add headers and match headers in our own VirtualService. Investigate.

Why

Generally speaking we should avoid touching the users configured resources.