ipa-kdb: common function to get key encodings/salt types
    
    This patch moves duplicate code in `ipadb_get_connection` to get default and
    supported key encodings/salt types from Kerberos container to a common
    function handling this task.
    
    It is actually a small cosmetic enhancement of the fix of
    https://fedorahosted.org/freeipa/ticket/4914
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>