ipalib: Expose krbPrincipalExpiration in CLI
    
    Adds a krbPrincipalExpiration attribute to the user class
    in user.py ipalib plugin as a DateTime parameter.
    
    Part of: https://fedorahosted.org/freeipa/ticket/3306
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>