Introduce a new namespace attribute to EntityTag

The idea being that tag name's could be re-used across namespaces, either
intentionally or accidentally.

Permissions / restrictions could also be applied to namespaces vs. individual
tag names.

Will need to sort out re-indexing and assign a proper namespace to all
existing tags.

Depends on spinnaker/front50#203

clouddriver-core/src/main/groovy/com/netflix/spinnaker/clouddriver/model/EntityTags.groovy

@@ -105,13 +105,18 @@ class EntityTags {
 
   static class EntityTag {
     String name
+    String namespace
     Object value
     EntityTagValueType valueType
 
     String getName() {
       return name.toLowerCase()
     }
 
+    String getNamespace() {
+      return (namespace ?: "default").toLowerCase()
+    }
+
     @JsonIgnore
     Object getValueForWrite(ObjectMapper objectMapper) {
       switch (valueType) {