Set the default Ceph CSI images as vars in the code instead of consts.
This allows these values to be overridden at build time with Go linker
-X flags. This allows users to build Rook in such a way that it will
automatically update the CSI images to a custom opinionated default
without having to manually manage the environment variable-based
overrides at upgrade time.
Signed-off-by: Blaine Gardner <blaine.gardner@suse.com>