WIP (untested): make syndicated.py use a slice-specific shared secret (part of issue #89)
Rename "syndicate" to "syndicate_service" to avoid import conflicts on OpenCloud.
Add "SliceSecret" model and admin.  This will store a secret shared between the observer and the syndicate daemons, on a per-slice basis.  It is automatically generated for the slice when a volume is attached to it.  Only the slice admin can read it.
This commit also adds volume filtering based on the user's capabilities (admin or not) and the volume's privacy settings.