Handle API limit exception in nova_utils.refresh_server

novaclient raises an OverLimit exception when a 413 response code (API
limit) is returned from the Nova API.  Previously, this would cause the
stack action to fail.  Log a warning instead.

Change-Id: I52c9dc96b34d0645662d98aaeebb7dabe6956e25
Implements: blueprint improve-api-polling
Merge "Handle API limit exception in nova_utils.refresh_server"