This commit adds a standalone setting.

It's well imaginable that you don't want to have a big
pilight network with all instances connected to each other.
In some situations, multiple standalone pilight instances
are therefor prefered. This commit adds the standalone setting.

When you set standalone to 1 pilight won't start it's ssdp
server and won't connect to other instances already running.
The downside is that you can't use ssdp discovery to connect
to your standalone instance.