Describe '[glance]/enable_rbd_download'

New option was introduced for faster download images directly from Ceph
instead of using the Glance API.

See the following blueprint for more details:

https://blueprints.launchpad.net/nova/+spec/nova-image-download-via-rbd

Compared to the blueprint, this new option was added instead of reviving
deprecated '[glance]/allowed_direct_url_schemes'.

Change-Id: Ib7e52f386df2134aa043dc703b378d0737b1d36d

Author: jsuchome

releasenotes/notes/enable_rbd_download-e60470890518a605.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    New ``[glance]/enable_rbd_download`` config option was introduced.
+    The option allows for the configuration of direct downloads
+    of Ceph hosted glance images into the libvirt image cache via rbd when
+    ``[glance]/enable_rbd_download= True`` and ``[glance]/rbd_user``,
+    ``[glance]/rbd_pool``, ``[glance]/rbd_connect_timeout``,
+    ``[glance]/rbd_ceph_conf`` are correctly configured.