This change allows compute hosts to quickly download and cache images on the
local compute host directly from Ceph rather than slow dowloads from the
Glance API.
New '[glance]/enable_rbd_download' option is introduced to enable this
behavior. This is slight change compared to the original idea described in
the relevant blueprint where it was discussed to use (now obsolete)
'[glance]/allowed_direct_url_schemes' option.
Additionally, when an image signature verification is requested, it should
be done also for the image fetched by the new download handler. This was
completely missing so far.
New '[glance]/rbd_{user,pool,ceph_conf,connect_timeout}' configurables
are introduced to allow operators to configure access to the cluster
hosting Glance without the need to use the existing '[libvirt]'
specific configurables. nova.storage.rbd_utils.RBDDriver has also been
modified to accept these but continues to default to the '[libvirt]'
specific configurables for now.
Change-Id: I3032bbe6bd2d6acc9ba0f0cac4d00ed4b4464ceb
Implements: blueprint nova-image-download-via-rbd
61aeb1adbc