wazo-chatd: presences are not initialized after machine reboot

Given I can GET correctly the presence of a user in wazo-chatd
Given Wazo Platform is installed on a Debian 10 machine with systemd
When I restart the machine hosting Wazo Platform
Then I cannot GET the presence of any user in wazo-chatd
Then I see the following error in `/var/log/wazo-chatd.log`
```
wazo-chatd.log:2020-09-14 17:02:05,047 [655] (ERROR) (xivo.rest_api_helpers): Presences are not initialized: {}
```

Expected: I can GET correctly the presence of a user in wazo-chatd