replica install: relax domain level check for promotion
    
    promote_check currently requires DL == 1.  Relax the check to
    require DL >= 1, so that things will work for future DL increases.
    
    Also separate the concerns of retrieving the current domain level,
    validating whether the domain level is supported by the IPA version,
    and validating whether the current domain level supports the replica
    installation method attempted (i.e. replica file versus promotion).
    
    Part of: https://fedorahosted.org/freeipa/ticket/5011
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>