A common issue is trying to run an AURO instance using the ext-net network.  I this case you may see the following error message come up during launch:

Error: Failed to launch instance Please try again later [Error: No valid host was found. Exceeded max scheduling attempts 3 for instance 1d0aa23a-23f4-4f17-9476-b2eb6a92d24d. Last exception: [u'Traceback (most recent call last):\n', u' File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 2039, in _do].

This is a general error, which actually means you need to use your internal network.  You cannot add the ext-net network to instances. Instead, using the internal or "Private" network.