Service: common method for service keytab requests
    
    Since creation of service principals and keytab retrieval are common
    operations, Service class should provide means to add service entry to LDAP,
    retrieve its keytab to designated destination and change the owner to service
    user.
    
    https://fedorahosted.org/freeipa/ticket/6405
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>