Handle API 500 response in OS::Nova::Server

novaclient raises a ClientException when a 500 response code is returned
from the Nova API.  Previously, this would cause the stack action to
fail.  Log a warning instead.

Closes-Bug: #1276517
Change-Id: I8f00d476c8d7166aca74fdcd3e0f2621c00d9802
Merge "Handle API 500 response in OS::Nova::Server"