tacker makes some configs failed in local.conf

[Jason]

https://github.com/openstack/tacker/blob/stable/mitaka/devstack/lib/tacker

In line 204, this line makes only $PUBLIC_PHYSICAL_NETWORK and $MGMT_PHYS_NET can use flat network and we can not create flat network in other networks.

In line 205, this line makes OVS_BRIDGE_MAPPINGS in local.conf for devstack become useless if someone needs to have bridge mapping configuration other than $PUBLIC_PHYSICAL_NETWORK:$PUBLIC_BRIDGE and$MGMT_PHYS_NET:$BR_MGMT.

Actually we can overwrite these two settings in local.conf:
[[post-config|/$Q_PLUGIN_CONF_FILE]]
[ovs]
But I think we should make it as easy as possible in local.conf.

Openstack version: Mitaka