[block storage] Let compute and block-storage cookbooks use image_api_chef_role

[Jay Pipes]

Reference: https://review.openstack.org/#/c/31534/1

We have a node["openstack"]["compute|block-storage"]["image_api_chef_role"] attribute that is currently unused in the nova.conf and cinder.conf templates. Instead, those templates use the endpoint "image-api" URI, which is great if the environment uses a load balancer to spread image API traffic across multiple servers, but the glance_api_servers config option in non-LB envs should list all Glance API servers in the cell/region/AZ so that the round-robin simple HA in Nova can make use of the multiple servers if one or more fail.

Solution is to remove the static call to endpoint "image-api" and subsequent variables injection and use the result of a config_by_role.