Changing the polling interval is not persistent

[ajoaoff]

Original issue opened by @italovalcy at kytos#62.

Hi guys,

If you change the polling interval and restart kytos, the change is lost.

Steps to reproduce:

1. Chante the polling interval on of_lldp:

$curl -X POST -d '{"polling_time": 1}' -H 'Content-type: application/json'  http://localhost:8181/api/kytos/of_lldp/v1/polling_time

2. Check if the polling interval was changed:

$ curl -s  http://localhost:8181/api/kytos/of_lldp/v1/polling_time
{"polling_time":1}

3. Restart kytos:

pkill kytosd
kytosd

4. Check the polling interval again

$ curl -s  http://localhost:8181/api/kytos/of_lldp/v1/polling_time

Expected behavior: the polling interval should be the same previously configured through the API.

Actual behavior: the polling interval falls back to the default value