pithos: Backend modification for enabling snapshot reconcilation

Modify pithos backend 'get_domain_objects' method
in order optionally (disabled by default)
to fetch all the objects of a specific domain
ignoring user permissions.

Related issue: #231
cyclades: Get images of all users

Make reconciliation and snf-manage {image, snapshot}-list commands, to
pass the 'check_permissions' argument to 'get_domain_objects' method of
PithosBackend in order to retrieve the images and snapshots of all
users, ignoring the user permission check.

Refs grnet/synnefo#231