Tidy Eth2Config generation at runtime

[paulhauner]

Description

Presently, when the lighthouse binary is started an Environment is created which contains an Eth2Config.

Then, this code ignores the environment.eth2_spec and replaces the Eth2Config in it's local context.

This means that there's no guarantee of consistency of the Eth2Config from an Environment.

Steps to resolve

• Pull out the code that generates the Eth2Config from CLI args (ArgMatches) (see [config.rs])(https://github.com/sigp/lighthouse/blob/eth1/beacon_node/src/config.rs) and place it in it's own function in the environment crate.

Notes

This issue is only valid after #542 is merged.

[ackintosh]

Hi, I'm addressing this issue, will file a PR to close this. 😃