Add check to validating admission webhook to reject EnvoyConfigs that manage an already existent node-id #160
Description
Add a check in the validating admission webhook for EnvoyConfig resources to avoid a new EnvoyConfig to be created for a node-id that is already being managed by an existent EnvoyConfig. This situation causes the discovery service to enter an infinite reconciliation loop as the EnvoyConfigRevisions match 2 (or more) different EnvoyConfigs.
This could also be fixed by forcing the node-id to be the same as the EnvoyConfig resource name, but this could break existing configurations.