Add a top level RUN_GANETI_DIR constant

This patch creates a base RUN_GANETI_DIR and then moves the other run
dir constants to use that (even if just setting BDEV_CACHE_DIR as equal
to it, rather than putting it deeper, for now).

Also we create a constant list of all the subdirs we need in RUN_DIR to
work properly, which we'll use when creating them in ganeti-noded.

Reviewed-by: iustinp