Networks should not be hard-coded

[jshufro]

Smartnode should ideally work for any ethereum network.

If we move network-specific constants and behavior into .yml files, we can trivially support locally run testnets, which will be important for e2e test automation. Additionally, chores like #437 become trivial- only one file needs to be updated, with optional "what's changed" update elsewhere.

We can package some 'default' networks with smartnode-install, and read an optional extra-networks.yml from the project directory (~/.rocketpool)

Edit: additionally, putting everything that fornax had to delete in

[activescott]

FYI: I am going to start investigating this one...