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