Manila support in Nova metadata service for share discovery & mounting

A highly sought feature for Manila (the Shared Filesystems service) is for client VMs to be able to automount file shares that are available to them. But a Nova instance is generally attached to the tenant data plane, not the cloud control plane. So there is no way for an instance to invoke Manila APIs to discover which shared filesystems are available for it to mount. The Nova metadata service offers an elegant solution to this sticky problem. By adding an endpoint to the metadata service, code running on a Nova instance can invoke that to get a list of shares available to that tenant context.

For example, the following URL invoked from within an instance:

http://169.254.169.254/openstack/latest/shares.json

can return enough data about shares available for that tenant/instance for the instance (via a cron job or cloud-init script) to mount the shares automatically. This was demonstrated live at the Vancouver summit.