Delete resources pool in reverse order

A release_lifo method is added to cloudcafe.common.resources.ResourcePool to
enable the deletion of resources in the opposite order in which they were
created. This is useful in certain scenarios, like testing Neutron, where the
the basic abstractions have to be created in the the sequence networks >
subnets > pools but have to be deleted in the opposite sequence

Change-Id: I774aaf6c24419680751620b7ba4cdcb2b038b015
Merge "Delete resources pool in reverse order"