Resolve host collisions in VirtualServer and Ingresses

[pleshakov]

Proposed changes

Previously, if a user deployed two Ingress resources with the same host, the Ingress controller would generate two NGINX servers with the same server_name. After the Ingress Controller reloaded NGINX, one virtual server would win that host, and the other one would be ignored by NGINX. No error was reported to the user in the events/status of the Ingress resources. In the IC logs, NGINX workers would report an error similar to this one 2020/07/31 22:23:57 [warn] 35#35: conflicting server name "cafe.example.com" on 0.0.0.0:80, ignored

The same problem occurred if a user deployed two VirtualServers with the same host or an Ingress and a VirtualServer with the same host.

This PR solves this problem:

• The Ingress Controller allows only one resource (Ingress or VirtualServer) to own a host
• If there are multiple resources with the same host, the oldest resource will win, and the rest will be ignored, the IC will report an error to their users by emitting an event and updating the statuses of the resources.