Add support for re-enrolling hosts using keytab
    
    A host that has been recreated  and does not have its
    host entry disabled or removed, can be re-enrolled using
    a previously backed up keytab file.
    
    A new option --keytab has been added to ipa-client-install. This
    can be used to specify path to the keytab and can be used instead
    of -p or -w options.
    
    A new option -f has been added to ipa-join. It forces client to
    join even if the host entry already exits. A new certificate,
    ssh keys are generated, ipaUniqueID stays the same.
    
    Design page: http://freeipa.org/page/V3/Client_install_using_keytab
    
    https://fedorahosted.org/freeipa/ticket/3374