Re-use API client for Heartbeat operations

Re-initialization of the API client causes the identification
of an older ironic API to be forgotten.

As such, we need to re-use the API client from the initial lookup
for the heartbeat.

Tested with current master branch and stable/mitaka ipa
in noauth mode, and was able to deploy without any issues.

Closes-Bug: #1616461
Change-Id: I3ae1e2ce83f614b5b936a3cbb1b9587c6c55a89a

ironic_python_agent/agent.py

@@ -88,8 +88,7 @@ class IronicPythonAgentHeartbeater(threading.Thread):
         """
         super(IronicPythonAgentHeartbeater, self).__init__()
         self.agent = agent
-        self.api = ironic_api_client.APIClient(agent.api_url,
-                                               agent.driver_name)
+        self.api = agent.api_client
         self.error_delay = self.initial_delay
         self.reader = None
         self.writer = None