check task_state in the live_migrate_instance

If server migration fails, the while loop will not break
until retry to 0,
we can check the task_state to avoid this situation.

Closes-Bug: #1728476
Change-Id: I07e1048eb736263a261456ee78c96fee9db13cb5

watcher/common/nova_helper.py

@@ -478,6 +478,9 @@ class NovaHelper(object):
                           'OS-EXT-SRV-ATTR:host') != dest_hostname \
                     and retry:
                 instance = self.nova.servers.get(instance.id)
+                if not getattr(instance, 'OS-EXT-STS:task_state'):
+                    LOG.debug("Instance task state: %s is null" % instance_id)
+                    break
                 LOG.debug(
                     'Waiting the migration of {0}  to {1}'.format(
                         instance,