Fix cluster behaviour with disabled file storage

There are a few issues with disabled file storage:
- cluster initialization is broken by default, as it uses the 'no'
  setting which is not a valid path
- some other parts of the code require the file storage dir to be a
  valid path; we workaround by skipping such code paths when it is
  disabled

A side effect is that we abstract the storage type checks into a
separate function and add validation in RepairNodeStorage (previously a
luxi client which didn't use cli.py and submitted an invalid type would
get "storage units of type 'foo' can not be repaired").

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>