Implement disabling of file-based storage

Rationale: the file-based storage backend can add/remove files under a
certain directory. However, the master node is also controlling the
setting of the file-based root directory, so basically it means we can't
prevent arbitrary modifications by the master of the node's filesystem.

In order to mitigate this for setups where the file-based storage is not
used, we introduce a new setting at ./configure time, that controls the
enable/disable of file-based storage. Since this is not modifiable by
the master (over RPC), it is now possible in this case to prevent
unintended modifications of the node's filesystem from the master.

The new setting is used in bdev.py to not expose the file-based storage
at all, and in cmdlib.py to prevent attempts at creation of such
instances.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>