Deprecate SolidFire Image Caching feature
Deprecate SolidFire Image Caching feature

We've had a general solution to image-caching in
Cinder for a while now and it works well, so we
can remove the SolidFire specific implementation
in favor of using the general solution.

https://docs.openstack.org/cinder/latest/\
admin/blockstorage-image-volume-cache.html

This patch marks the SolidFire config option
as deprecated, and we'll remove the feature
altogether in the next release.

Change-Id: Iefc207955e4718a7d9c3b235b133a4c629977d03

openstack/cinder,phenoxim/cinder,Datera/cinder,mahak/cinder,j-griffith/cinder,openstack/cinder,Datera/cinder,phenoxim/cinder,j-griffith/cinder,mahak/cinder