grizzly: direct file copy from glance

[Tom Fifield]

https://review.openstack.org/#/c/19408/ introduced a new feature to nova: Directly copy a file URL from glance.

That patch allows nova to use shutil to copy a file directly from the
glance store. For this to work glance needs to enable direct_url
descriptions in the image metadata. This is done by setting
show_image_direct_url to True in glance-api.conf. The path used to back
glance also must match the path used to back nova instance storage. This
should be a large performance increase for the cases where both glance
and nova instance storage are backed by a shared file system like gluster
or ceph.

A new option has been added to nova: allowed_direct_url_schemes
This option is set to a list of schemes that can be directly accessed.
For this first simple patch only file:// is valid, and the
implementation is simple and inline. In the future it would be better
to make this into propagation modules which are dynamically loaded
according to the scheme.

62 cfg.ListOpt('allowed_direct_url_schemes',
   63 default=[],
   64 help='A list of url scheme that can be downloaded directly '
   65 'via the direct_url. Currently supported schemes: '
   66 '[file].'),
   67 ]

This new feature should be documented and the config option added to the reference