Use tenancity to backoff when layer is locked
Use tenancity to backoff when layer is locked

Rather than doing a while loop and sleep, let's use tenancity to do an
exponential backoff off when the layer is already being fetched by
another process.

Change-Id: I47c9a2693d831958958fa6ea7cccbdd8fdb12277

openstack/tripleo-common,openstack/tripleo-common