[warm-reboot-fib] Why use reboot command instead of warm-reboot

Description Some questions about this test:

Why use reboot command instead of warm-reboot in warm-reboot-fib.yml?
Test will be hangs on reboot step, whether the step should be performed asynchronously?
Fib test finished in 12s on T1 topology, the flow is not verified to the entire warm-reboot process.

Additional information you deem important:

```
TASK [test : debug] ************************************************************
task path: /var/root/sonic-mgmt/ansible/roles/test/tasks/warm-reboot-fib.yml:72
Friday 15 February 2019  04:08:35 +0000 (0:00:00.146)       0:00:25.564 ******* 
ok: [str-msn2700-01] => {
"msg": "Result of Fib {u'changed': True, u'end': u'2019-02-15 04:08:34.954594', u'ansible_job_id': 
u'875803383533.450', u'stdout': u'WARNING: No route found for IPv6 destination :: (no default 
route?)\\nfib_test.FibTest ... ok\\n\\n---------------------------------------------------------------------- 
\\nRan 1 test in 11.282s\\n\\nOK', u'cmd': u'ptf --test-dir ptftests fib_test.FibTest   --platform-dir 
ptftests  --platform remote  -t \"testbed_type=\\'t1\\';router_mac=\\'3c:2c:99:c4:81:2a\\';fib_info=\\'/root/fib_info.txt\\';ipv4=True;ipv6=True\"  --relax --debug info --log-file /tmp/fib_test.FibTest.ipv4.True.ipv6.True.2019-02-15-04:08:22.log  --disable-vxlan --disable-geneve --disable-erspan --disable-mpls --disable-nvgre 2>&1', u'start': u'2019-02-15 04:08:22.306614', u'finished': 1, u'stderr': u'', u'rc': 0, u'delta': u'0:00:12.647980', 'stdout_lines': [u'WARNING: No route found for IPv6 destination :: (no default route?)', u'fib_test.FibTest ... ok', u'', u'----------------------------------------------------------------------', u'Ran 1 test in 11.282s', u'', u'OK'], u'warnings': []}"
}
```

sonic-net / sonic-mgmt

[warm-reboot-fib] Why use reboot command instead of warm-reboot #811