Query errata repo IDs
    
    Besides the new method get_pulp_repository_ids, also refactor
    _errata_get, because Errata has to deal with two kinds of Errata APIs, a
    REST-style API and legacy HTTP API. The former one must have version
    information api/v1 as prefix, whereas legacy HTTP API does not need it.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>