Allow wait_for_image_status() to have multiple

This lets us wait for one of multiple target states so we can more
effectively avoid race conditions where we miss an intermediate state
and first poll the final one.

Change-Id: If12c35d38c03fa329ebbf536e2fcd3ff2bd136de

releasenotes/notes/image-wait-multiple-79c55305b584b1ba.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    The wait_for_image_status() waiter now allows a list of status values
+    instead of just a string, and returns the state the image was in when we
+    stopped waiting.

tempest/common/waiters.py

@@ -154,13 +154,21 @@ def wait_for_server_termination(client, server_id, ignore_error=False,
 
 
 def wait_for_image_status(client, image_id, status):
-    """Waits for an image to reach a given status.
+    """Waits for an image to reach a given status (or list of them).
 
     The client should have a show_image(image_id) method to get the image.
     The client should also have build_interval and build_timeout attributes.
+
+    status can be either a string or a list of strings that constitute a
+    terminal state that we will return.
     """
     show_image = client.show_image
 
+    if isinstance(status, str):
+        terminal_status = [status]
+    else:
+        terminal_status = status
+
     current_status = 'An unknown status'
     start = int(time.time())
     while int(time.time()) - start < client.build_timeout:
@@ -171,8 +179,8 @@ def wait_for_image_status(client, image_id, status):
             image = image['image']
 
         current_status = image['status']
-        if current_status == status:
-            return
+        if current_status in terminal_status:
+            return current_status
         if current_status.lower() == 'killed':
             raise exceptions.ImageKilledException(image_id=image_id,
                                                   status=status)
@@ -184,7 +192,7 @@ def wait_for_image_status(client, image_id, status):
     message = ('Image %(image_id)s failed to reach %(status)s state '
                '(current state %(current_status)s) within the required '
                'time (%(timeout)s s).' % {'image_id': image_id,
-                                          'status': status,
+                                          'status': ','.join(terminal_status),
                                           'current_status': current_status,
                                           'timeout': client.build_timeout})
     caller = test_utils.find_test_caller()