Fix revert on 404 from amphora agent startup

When the amphora agent is starting up, it will return 404 until
the startup is complete.  This patch adds one retry when the driver
receives a 404 back from the agent to allow time for it to finish
starting.

Change-Id: Idcf666dc508d77fb1eb1d46acdaa1b4afa278b8c
Closes-Bug: #1631525

octavia/amphorae/drivers/haproxy/rest_api_driver.py

@@ -257,6 +257,7 @@ class AmphoraAPIClient(object):
 
         headers['User-Agent'] = OCTAVIA_API_CLIENT
         self.ssl_adapter.uuid = amp.id
+        retry_attempt = False
         # Keep retrying
         for a in six.moves.xrange(CONF.haproxy_amphora.connection_max_retries):
             try:
@@ -268,6 +269,11 @@ class AmphoraAPIClient(object):
                     r = _request(**reqargs)
                 LOG.debug("Connected to amphora. Response: {resp}".format(
                     resp=r))
+                # Give a 404 response one retry.  Flask/werkzeug is
+                # returning 404 on startup.
+                if r.status_code == 404 and retry_attempt is False:
+                    retry_attempt = True
+                    raise requests.ConnectionError
                 return r
             except (requests.ConnectionError, requests.Timeout):
                 LOG.warning(_LW("Could not connect to instance. Retrying."))