Allow host re-enrollment using delegation

A new option --force-join has been added to ipa-client-install.
It forces the host enrollment even if the host entry exists.
Old certificate is revoked, new certificate and ssh key pair
generated. See the relevant design for the re-enrollment part:
http://freeipa.org/page/V3/Forced_client_re-enrollment

https://fedorahosted.org/freeipa/ticket/3482
Add logging to join command

The following is mentioned in the log now:
  - existence of host entry (if it already does exist)
  - missing krbprincipalname and its new value (if there was no
    principal name set)

https://fedorahosted.org/freeipa/ticket/3481