Use '_get_domain_objects' function of Pithos backend, to list all
objects in a 'plankton' domain, instead of looping over all accessible
accounts, containers and objects. This function also returns metadata
and permissions for each object in order to avoid two extra calls for
each returned image.

This should provide a significant performance boost in listing available
images.

Note: Currently plankton API filters in 'list_images' are not
implemented.

Refs #3510