In concurrent execution cases a failure can occur in the Neutron
IPAM component due to failure to obtaining a lock. In such cases
the IPAM component raises a retry exception which should be not
be eaten, but relayed as is so that the operation can be retried.
This patch checks if the exception raised during a subnet allocation
from a subnetpool fails due to a retry exception, and if so, raises
it as is to facilitate a retry.
Change-Id: I381cdf533b27d710f68903f0cfb516043b4607d6