OpenStack: Optimize cluster deletion

Previously each destroy function would exit on the first conflict and
it was expected to retry on a next iteration, hoping that in the mean
time the conflict that prevented removal of the resource was fixed.
While this strategy works, it is also slower then necessary. A better
strategy is to try deleting all resources and ignore the ones that have
conflicts. On the next iteration there will be less conflicts.

This patch was tested on a openstack cluster with openshiftSDN, and
we observed a 37% faster cluster deletion. I expect the performance
boost to be significantly higher for clusters using kuryr.