Adds a tripleo-nova-image-cache role to manage the nova local image cache on
remote compute nodes.
For multi-site/stack deployments the image can be downloaded once and
distributed to the remaining nodes in the stack to minimise WAN traffic.
Also adds a playbook to run the role using the single/multi-stack inventory
and the overcloudrc environment variables.
Change-Id: Ib5aaa22f6cf307181d8f34cf89f9f24619b43004
Implements: blueprint tripleo-nova-cache-mgmt
Oliver Walsh