support for default_backend (#222)

* support for setting a service to the default_backend rule

* docs

* renamed serviceDefaultBackend to isDefaultBackend

* removed duplicate

* fix

* added integration test and changed env to IS_DEFAULT_BACKEND

* moved test

* Update integration_swarm_test.go

integration_tests/integration_swarm_test.go

@@ -293,7 +293,7 @@ func (s IntegrationSwarmTestSuite) Test_ReconfigureFromEnvVars() {
 }
 
 func (s IntegrationSwarmTestSuite) Test_ReconfigureWithDefaultBackend() {
-	params := "serviceName=go-demo&servicePath=/xxx&port=8080"
+	params := "serviceName=go-demo&servicePath=/xxx&port=8080&isDefaultBackend=true"
 	s.reconfigureService(params)
 
 	resp, err := s.sendHelloRequest()