Mark optional DNS record parts
    
    All DNS record part options in dnsrecord commands need to be
    optional so that all of them are not required in every dnsrecord
    command. However, FreeIPA API then does not include an information
    which DNS record part options are optional in term of creating
    a new DNS record. For example, LOC record option "latitude seconds"
    is not needed to add a new LOC record.
    
    This patch adds a flag "dnsrecord_optional" to all such options so
    that this information is available for any other UI reading the
    FreeIPA API.
    
    https://fedorahosted.org/freeipa/ticket/2208