Remove importance of order of rule in RestoreSession (#795)

Tasks:
- [x] Remove importance of rule order
- [x] Add validator to ensure followings,
	- [x] There is at most one rule with empty targetHosts field.
	- [x] No two rule with non-empty targetHosts matches for a host.
	- [x] If snapshot is specified in a rule then paths is not specified.
- [x] Update concept doc to make rules behavior clear

Fixes: stashed/stash#790

server/start.go

@@ -86,6 +86,7 @@ func (o StashOptions) Config() (*server.StashConfig, error) {
 		"/apis/admission.stash.appscode.com/v1alpha1/replicationcontrollermutators",
 		"/apis/admission.stash.appscode.com/v1alpha1/replicasetmutators",
 		"/apis/admission.stash.appscode.com/v1alpha1/deploymentconfigmutators",
+		"/apis/admission.stash.appscode.com/v1beta1/restoresessionvalidators",
 	}
 
 	extraConfig := controller.NewConfig(serverConfig.ClientConfig)