crd: require unique hosts

Example validation message:
```
Error from server (Invalid): error when creating "rg.yaml": RouteGroup.zalando.org "duplicate-hosts" is invalid: spec.hosts[1]: Duplicate value: "foo.example.org"
```

Signed-off-by: Alexander Yastrebov <alexander.yastrebov@zalando.de>

apis/zalando.org/v1/types.go

@@ -43,6 +43,7 @@ type RouteGroupList struct {
 type RouteGroupSpec struct {
 	// List of hostnames for the RouteGroup
 	// +kubebuilder:validation:MinItems=1
+	// +listType=set
 	Hosts []string `json:"hosts,omitempty"`
 	// List of backends that can be referenced in the routes
 	Backends []RouteGroupBackend `json:"backends"`
@@ -179,6 +180,7 @@ type RouteGroupTLSSpec struct {
 	// The values in this list must match the host name(s) used for
 	// the RouteGroup in order to terminate TLS for the host(s).
 	// +kubebuilder:validation:MinItems=1
+	// +listType=set
 	Hosts []string `json:"hosts"`
 
 	// SecretName is the name of the secret used to terminate TLS traffic.

zalando.org_routegroups.yaml

@@ -136,6 +136,7 @@ spec:
                   type: string
                 minItems: 1
                 type: array
+                x-kubernetes-list-type: set
               routes:
                 description: Routes describe how a matching HTTP request is handled
                   and where it is forwarded to
@@ -221,6 +222,7 @@ spec:
                         type: string
                       minItems: 1
                       type: array
+                      x-kubernetes-list-type: set
                     secretName:
                       description: |-
                         SecretName is the name of the secret used to terminate TLS traffic.