Quick fix of a bug in lb_create method in lbaas driver

In recent patch, the input parameter list of lb_create method in
lbaas driver was changed. This will fail existing invokings of this
function. This patch proposes a fix for it.

Change-Id: I734d5032add067ec5963d6ae7a6281868d484496

senlin/drivers/openstack/lbaas.py

@@ -80,7 +80,7 @@ class LoadBalancerDriver(base.DriverBase):
 
         return False
 
-    def lb_create(self, vip, pool, hm):
+    def lb_create(self, vip, pool, hm=None):
         """Create a LBaaS instance
 
         :param vip: A dict containing the properties for the VIP;
@@ -162,6 +162,9 @@ class LoadBalancerDriver(base.DriverBase):
             _cleanup(msg, **result)
             return res, msg
 
+        if not hm:
+            return True, result
+
         # Create health monitor
         try:
             health_monitor = self.nc().healthmonitor_create(