Support the use of an external database setup

[peterklijn]

Hi,

It would be great if this repository would support the setup of K3s using an external database, in addition to the single server setup using embedded SQLite and the multi server setup using the embedded etcd store.

I made a POC of how this would work here:
#339

It misses documentation, but works otherwise, so I hope it is sufficient to get some feedback.

Thanks