Adds FLAT network type

Fix proposed to branch: master
Review: https://review.openstack.org/492042

When we allow FLAT type, then we are getting the following test case error.
We need to look into the error in more detail.

ERROR [oslo_db.sqlalchemy.exc_filters] DBAPIError exception wrapped from (sqlite3.InterfaceError) Error binding parameter 0 - probably unsupported type. [SQL: u'SELECT ports.project_id AS ports_project_id, ports.id AS ports_id, ports.name AS ports_name, ports.network_id AS ports_network_id, ports.mac_address AS ports_mac_address, ports.admin_state_up AS ports_admin_state_up, ports.status AS ports_status, ports.device_id AS ports_device_id, ports.device_owner AS ports_device_owner, ports.ip_allocation AS ports_ip_allocation, ports.standard_attr_id AS ports_standard_attr_id, standardattributes_1.id AS standardattributes_1_id, standardattributes_1.resource_type AS standardattributes_1_resource_type, standardattributes_1.description AS standardattributes_1_description, standardattributes_1.revision_number AS standardattributes_1_revision_number, standardattributes_1.created_at AS standardattributes_1_created_at, standardattributes_1.updated_at AS standardattributes_1_updated_at, standardattributes_2.id AS standardattributes_2_id, standardattributes_2.resource_type AS standardattributes_2_resource_type, standardattributes_2.description AS standardattributes_2_description, standardattributes_2.revision_number AS standardattributes_2_revision_number, standardattributes_2.created_at AS standardattributes_2_created_at, standardattributes_2.updated_at AS standardattributes_2_updated_at, trunks_1.project_id AS trunks_1_project_id, trunks_1.id AS trunks_1_id, trunks_1.admin_state_up AS trunks_1_admin_state_up, trunks_1.name AS trunks_1_name, trunks_1.port_id AS trunks_1_port_id, trunks_1.status AS trunks_1_status, trunks_1.standard_attr_id AS trunks_1_standard_attr_id, portsecuritybindings_1.port_id AS portsecuritybindings_1_port_id, portsecuritybindings_1.port_security_enabled AS portsecuritybindings_1_port_security_enabled, portdnses_1.port_id AS portdnses_1_port_id, portdnses_1.current_dns_name AS portdnses_1_current_dns_name, portdnses_1.current_dns_domain AS portdnses_1_current_dns_domain, portdnses_1.previous_dns_name AS portdnses_1_previous_dns_name, portdnses_1.previous_dns_domain AS portdnses_1_previous_dns_domain, portdnses_1.dns_name AS portdnses_1_dns_name, portdnses_1.dns_domain AS portdnses_1_dns_domain, securitygroupportbindings_1.port_id AS securitygroupportbindings_1_port_id, securitygroupportbindings_1.security_group_id AS securitygroupportbindings_1_security_group_id, qos_port_policy_bindings_1.policy_id AS qos_port_policy_bindings_1_policy_id, qos_port_policy_bindings_1.port_id AS qos_port_policy_bindings_1_port_id, portbindingports_1.port_id AS portbindingports_1_port_id, portbindingports_1.host AS portbindingports_1_host, ml2_port_bindings_1.port_id AS ml2_port_bindings_1_port_id, ml2_port_bindings_1.host AS ml2_port_bindings_1_host, ml2_port_bindings_1.vnic_type AS ml2_port_bindings_1_vnic_type, ml2_port_bindings_1.profile AS ml2_port_bindings_1_profile, ml2_port_bindings_1.vif_type AS ml2_port_bindings_1_vif_type, ml2_port_bindings_1.vif_details AS ml2_port_bindings_1_vif_details, ml2_port_bindings_1.status A...

Change abandoned by Sangho Shin (<email address hidden>) on branch: master
Review: https://review.openstack.org/492042
Reason: FLAT type is not supported.

It seems that FLAT type is not supported.

Fix proposed to branch: stable/mitaka
Review: https://review.openstack.org/574121

Change abandoned by Sangho Shin (<email address hidden>) on branch: stable/mitaka
Review: https://review.openstack.org/574121
Reason: Need to add other patches for tox tests