pithos: Improve test

Extend test for listing objects until a specific timestamp
to cover the case that an object is deleted after the provided timestamp.
pithos: PEP8 fix
pithos: Reset negative object sizes

Provide a database migration script that discovers
erroneously declared negative objects sizes and sets
them to 0.
pithos: Raise custom exception for invalid policies.

Raise 'InvalidPolicy' instead of 'ValuerError' exception
during account/container updates.
pithos: Change PithosBackendPool signature

PithosBackendPool constructor used to accept a number of keyword arguments
that it would pass them to the ModularBackend during the backend
instantiation.
Hence, this code required refactoring each time a new backend argument
was introduced.
This commit changes the PithosBackendPool to accept
a kwargs dictionary instead.
pithos: Recover 'get_domain_objects' from error case

Method 'get_domain_objects' should not fail if an object
(snapshot) is not available.
pithos: Fix sqlite backend
cyclades: Fix small issues with snapshots

* fix IndexError in snf-ganeti-event when processing an
  OP_INSTANCE_SNAPSHOT operation without defining --disk option.
* fix TypeError in snf-dispatcher when processing an
  OP_INSTANCE_SNAPSHOT without snapshot information.

Both issues are caused by using directly the 'gnt-instance snaspshot'
command.
gtools: Make eventd tolerate unhandled exceptions

Make 'snf-ganeti-eventd' daemon tolerate exceptions while processing
events, in order to avoid losing many Ganeti events because of a failure
to process a message of specific type.