implement get_security_groups_count

currently the neutron_plugin_contrail can't get count for security_group and security_group_rule, because the code just return 0, see below

neutron_plugin_contrail/plugins/opencontrail/contrail_plugin_base.py

    def get_security_groups_count(self, context, filters=None):
        return 0

    def get_security_group_rules_count(self, context, filters=None):
        return 0

get the count for security_group and security_group_rule is importance for quota, this patch going to make the count work