In the pre-deploy step of our neutron-arista tests we download an
Arista image from our swift bucket. It sometimes fails when our CI
system is loaded. This patch adds a tenacity retry.
When creating an Arista-backed Neutron network, the API call from Neutron
to Arista sometimes spuriously fails. This is not fatal and Neutron
simply retries a few seconds later. The tests now account for that.
Aurelien Lourot