'API Version 2015-10-01 defines the following filters for DescribeDhcpOptions:
* dhcp-options-id
* key
* value
* tag:key=value
* tag-key
* tag-value
Taken from: http://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeDhcpOptions.html'

def get_filter_value(self, filter_name):

    if (filter_name == u'dhcp-options-id'):
        return self.id
    elif (filter_name == u'key'):
        return list(self._options.keys())
    elif (filter_name == u'value'):
        values = [item for item in list(self._options.values()) if item]
        return itertools.chain(*values)
    filter_value = super(DHCPOptionsSet, self).get_filter_value(filter_name)
    if (filter_value is None):
        self.ec2_backend.raise_not_implemented_error(u"The    filter    '{0}'    for    DescribeDhcpOptions".format(filter_name))
    return filter_value
