Fix Haskell compatibility tests with disabled file storage

When file storage is disabled at ./configure time, we shouldn't pass
opcodes containing DTFile/DTSharedFile/StorageFile to Python for
validation, as they will fail.

This patch implements this by simply tweaking the Arbitrary instances
for DiskTemplate and StorageType (which IMHO is a nice and clean
way!), and also fixing the generation of arbitrary IPolicies to use
the correct 'allDiskTemplates' list (otherwise we'd loop forever
trying to generate a list of all templates).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>