feat(publicip): PUBLICIP_ENABLED replaces PUBLICIP_PERIOD

- No point periodically fetch the public IP address. Could not find anything mentioning why this was added.
- Simplification of the publicip loop code
- `PUBLICIP_ENABLED` (on, off) can be set to enable or not public ip data fetching on VPN connection
- `PUBLICIP_PERIOD=0` still works to indicate to disable public ip fetching
- `PUBLICIP_PERIOD` != 0 means to enable public ip fetching
- Warnings logged when using `PUBLICIP_PERIOD`

Dockerfile

@@ -204,7 +204,7 @@ ENV VPN_SERVICE_PROVIDER=pia \
     UPDATER_VPN_SERVICE_PROVIDERS= \
     # Public IP
     PUBLICIP_FILE="/tmp/gluetun/ip" \
-    PUBLICIP_PERIOD=12h \
+    PUBLICIP_ENABLED=on \
     PUBLICIP_API=ipinfo \
     PUBLICIP_API_TOKEN= \
     # Storage