Splice lava-docker's boards.yaml into master/slave yaml. Allow multiple labs in master conf.

will also update documentation for "slave" field of each board.

a) will add zmq_auth folders

Let's go by example:

master.yaml

a) .... how do we add the slave zmq public keys there if we setup the master only.

b) .... how do we deal with multiple slave labs and thus multiple zmq public keys

slave.yaml

c) how to handle master pubkey in slave.yaml

d) how to assign boards per slave (if multiple slaves are defined in slaves section)

How are the zmq keys and so on split then between the files ?

The `lavalab-gen[sh|py]` tool now takes an optional argument which can be the YAML file.   

So, you can create a `master.yaml` and run `./lavalab-gen.py master.yaml` and a separate `slaves.yaml` and run `lavalabgen.py slaves.yaml`.