Path map may disable flow timeout incorrectly.

If a flow is mapped by path map, and flow timeout is not specified in the path map,
flow timeout setting should be inherited from the virtual tenant configuration.
But zero is set in flow entries incorrectly.

How to reproduce:

1. Start VTN Manager and mininet.

2. Create a VTN and vBridge, and map all untagged networks to that vBridge using
VLAN mapping.

POST /controller/nb/v2/vtn/default/vtns/vtn_1
Request body:

{"idleTimeout": 300}

POST /controller/nb/v2/vtn/default/vtns/vtn_1/vbridges/bridge_1
Request body: {}

POST /controller/nb/v2/vtn/default/vtns/vtn_1/vbridges/bridge_1/vlanmaps
Request body: {}

3. Configure path map without specifying idle/hard timeout, and map all flows
to the path map.

PUT /controller/nb/v2/vtn/default/flowconditions/all
Request body: {}

PUT /controller/nb/v2/vtn/default/pathpolicies/1
Request body:

{"default": 10}

PUT /controller/nb/v2/vtn/default/vtns/vtn_1/pathmaps/1
Request body:

{"condition": "all", "policy": 1}

4. Send a packet.

mininet> h1 ping h2

Then VTN Manager will install flow entries like this:

cookie=0x0, duration=6.029s, table=0, n_packets=5, n_bytes=434,
priority=10,in_port=1,vlan_tci=0x0000/0x1fff,
dl_src=f2:89:2d:8c:8e:c9,
dl_dst=a6:76:f3:12:9b:ca actions=output:2

Although idle timeout value should be 300 as configured in the VTN configuration,
zero will be set.