Core shared file storage support

This patch introduces core file storage support, consisting of the following:

A configure-time switch for enabling/disabling shared file storage
support and controlling the shared file storage location:
--with-shared-file-storage-dir=.  Shared file storage configuration is then
available as _autoconf.ENABLE_SHARED_FILE_STORAGE and
_autoconf.SHARED_FILE_STORAGE_DIR and there is a cluster-wide ssconf
key named "shared_file_storage_dir" for changing the file location.

A new disk template named "sharedfile" (DT_SHARED_FILE), using
ganeti.bdev.FileStorage.

Auxiliary functions in lib/config.py to handle shared file storage.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
[iustin@google.com: small style fixes]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>