Detach leap-node from the root network provider url

[troggy]

We want people to run the node using their own providers. Now, when you spin up a testnet node it will take a root network id from the config, but still will use hardcoded provider URL (infura).

Scope:

• add an optional providerUrl cli argument
• on start, check that given provider is for the same network (compare with rootNetworkId from the network config)
• if not specifed, fallback to LeapDAO's Infura (questionable, should we do that?)

Size:
XXS

[troggy]

question here: should we keep infura urls in the repo at all.

Pros: easier for us to start the nodes
Cons: no incentive to use your own infrastructure

[pinkiebell]

question here: should we keep infura urls in the repo at all.

I say no. Let's remove them. Not that someone uses it and we change the infura account and someone get's bite'n by that.

[troggy]

via #352