Multiple networks support and a few readability tweaks

[dimitern]

Order of arguments to kvm-maas-add-node has been altered to put
networks (one or more) at the end. For kvm-maas-remove-node, a
list of networks is accepted for compatibility, but it's ignored.

[dimitern]

@frobware So how about this PR ? I already have a couple of ideas for follow-ups :)

[frobware]

There's an issue with the order of networks - the first should be the PXE network. I'll merge and fixup afterwards.

[frobware]

Thanks and apologies for the delay.