Allow multiple instances without hostname conflicts

[lukechilds]

Add a random hex prefix to the hostname so multiple instances can be run simultaneously without conflicting.

Use openssl:

$ openssl rand -hex 4
d67dd434

So hostnames would be:
ac50c42d-umbrel-dev.local
70b5e8c2-umbrel-dev.local
a9c57fa4-umbrel-dev.local
etc

[nolim1t]

That would be awesome... you could connect them all up and open channels with each other too.

[lukechilds]

Yeah it would be really useful for creating regtest networks.