Add shared file storage design doc

Add doc/design-shared-storage.rst to document the proposed changes and update
Makefile.am respectively.

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