consul: Do not deregister external checks

This commit causes sync to skip deregistering checks that are not
managed by nomad, such as service maintenance mode checks.  This is
handled in the same way as service registrations - by doing a Nomad
specific prefix match.

command/agent/consul/client.go

@@ -513,7 +513,7 @@ func (c *ServiceClient) sync() error {
 		// Nomad managed checks if this is not a client agent.
 		// This is to prevent server agents from removing checks
 		// registered by client agents
-		if !isNomadService(check.ServiceID) || !c.isClientAgent {
+		if !isNomadService(check.ServiceID) || !c.isClientAgent || !isNomadCheck(check.CheckID) {
 			// Service not managed by Nomad, skip
 			continue
 		}
@@ -1182,6 +1182,12 @@ func createCheckReg(serviceID, checkID string, check *structs.ServiceCheck, host
 	return &chkReg, nil
 }
 
+// isNomadCheck returns true if the ID matches the pattern of a Nomad managed
+// check.
+func isNomadCheck(id string) bool {
+	return strings.HasPrefix(id, nomadCheckPrefix)
+}
+
 // isNomadService returns true if the ID matches the pattern of a Nomad managed
 // service (new or old formats). Agent services return false as independent
 // client and server agents may be running on the same machine. #2827