un"hardcode" setting chain configuration

[meowsbits]

this needs improving:

https://github.com/etclabscore/multi-geth/blob/master/cmd/utils/flags.go#L1567-L1623

having "hardcoded" switches for default chain configs everywhere is too complex and too redundant and leaves too many opportunities for errors and inconsistencies:

• no mordor: https://github.com/etclabscore/multi-geth/blob/master/cmd/faucet/faucet.go#L119
• missing chains: https://github.com/etclabscore/multi-geth/blob/master/mobile/geth.go#L143-L166

much of this is because of crufty, brittle code (inheriting code where these cases have been carelessly hardcoded with approaching infinite opinionation).

I'm not sure the best pattern for approach yet. Big global? Moar interfaces? Mmmm..

Rel #63

[meowsbits]

Changes should (eventually):

• Provide a LIST of available default chain configurations
• Read default chain configurations from JSON files instead of having them hardcoded in Go.