cyclades: Use bulk create in DB migration

Speed up DB migration for volumes by using bulk creation. Also fix
backwards migration.
cyclades: Automatically adopt unknown Ganeti disks

Update 'snf-dispatcher' and 'reconciliation' mechanism to automatically
adopt unknown disks of Ganeti instances. Adoption is performed by
creating a Cyclades Volume and then renaming the instance's disk with
the volume's uuid (snf-vol-%id). The quotas for the new volume are
charged to the server's project.
cyclades: Remove volume-import management command

Remove unnecessary command, since unknown Ganeti disks are automatically
adopted by snf-dispatcher and reconciliation mechanism.
cyclades: Fix corner case when updating vm disks

Fix a corner case when updating the disks of an instance, in
snf-dispatcher and reconciliation. This corner case is when a VM has both:

* unknown Ganeti instances (without a unique Synnefo ID)
* volumes that exist in DB but not in Ganeti

Such a case cannot be fixed automatically, and should be resolved by the
administrator. This case will also appear during the upgrade to version
0.16.
cyclades: Fix not null constraint

Image's 'os' and 'osfamily' attributes cannot be null.
cyclades: Fix bug in string formatting
cyclades: Move snapshot registration to plankton

Move the code that registers a snapshot to Pithos (using the
'register_object_map' method of the Pithos backend) to plankton.