Make ganeti-noded create BDEV_CACHE_DIR automatically

Currently in order to deal with tmpfs /var/run, we create the
BDEV_CACHE_DIR in the init script. However, that does not cover all the
cases, and it's not a proper place to deal with it: for example, dealing
with not initialized clusters and the master node is more complicated.

Therefore, this patch does:
  - make ganeti-noded create the directory automatically
  - make ganeti-noded error out if it can't create it or it's already
    there but not a directory
  - remove the creation from the init.d script

Reviewed-by: ultrotter