Rename obj_type to bundle_type
'obj_type' was confusing, because it means 'the type of objects in the vault backend', which is actually made of an object type (dir/rev/...) AND the cooker name.
Plus, I want to reuse the name 'obj_type' in a future commit to reference what is actually the object type, in the swh-model definition.
Migrated from D6101 (view on Phabricator)