Nova cache management playbook

Related to https://blueprints.launchpad.net/tripleo/+spec/split-control-plane-unified-inventory

For smaller edge sites (without a local glance cache) create ansible playbooks to manage the image cache on the local filesystem of each compute node.
Ideally images should be downloaded over http (e.g using glance client) as ssh is severely affected by network latency.
We can potentially optimise the image transfer to minimise WAN bandwidth usage by downloading to a single compute at each site and distributing from there to all neighbouring computes.