Remove Glance cache

Our default backend is RBD, using the 'keystone+cachemanagement' is
extremely dangerous since it will result in images getting cached in the
controllers under /var/lib/glance/image-cache/ and will potentially lead to
a full system sooner or later. Moreover since we use RBD CoW clone there
is no point to cache anything. Everytime a virtual machine is booted
often, this image will go into the cache. Think about Windows 50GB big
images, now think about 10 of them :D.

Going further if we have to use another backend I won't recommend using
it. I only Swift as a suitable backend for this, but regarding some
recent effort on the Nova side (image direct url) to get the image
directly from the storage entity I don't think it's useful to have any
images cached anyways.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>