Add direct download feature for rbd glance backend

This blueprint aims to support specifying the direct download feature for "rbd glance backend" when nova boot.

[Background]
Supporting for direct downloads of glance locations is already added by plug-in modules.
(https://review.openstack.org/37817)

And this feature only can support file not rbd images.
Added new rbd download scheme for Nova. With that change
Nova can download RBD images directly from Ceph using 'rbd export'
command not involving rather slow download by HTTP from Glance
server.

[Benefit for the user]
That makes image download process much faster if local
disks storage is configured for Nova.

[Usage]
Option must be set in nova.conf:
allowed_direct_url_schemes = rbd