Fix installing server with external CA
    
    Reorganize ipa-server-instal so that DS (and NTP server) installation
    only happens in step one.
    
    Change CAInstance to behave correctly in two-step install.
    
    Add an `init_info` method to DSInstance that includes common
    attribute/sub_dict initialization from create_instance and create_replica.
    Use it in ipa-server-install to get a properly configured DSInstance
    for later tasks.
    
    https://fedorahosted.org/freeipa/ticket/3459