Ticket 48771 - lib389 - get ns-slapd version
    
    Bug description:
    Some tests should be executed only with 389-ds-base
    that implements required features/has fixes. This can be done with
    py.test skipif fixture. But we don't have a way to get the version
    of ns-slapd that is currently used.
    
    Fix description:
    Add functions get_ds_version() and ds_is_older().
    get_ds_version() returns a string like "1.3.4.8 B2016.043.2254"
    ds_is_older(version) returns boolean value if the current version is
    older than provided string.
    
    https://fedorahosted.org/389/ticket/48771
    
    Author: vashirov
    
    Reviewed by: wibrown, mreynolds (Thanks!)