Script to return Ethereum address from a private key

[akolotov]

In order to provide configuration for the token bridge watchers, the deployment playbook needs to calculate an Ethereum address from a private key (for more details see https://github.com/poanetwork/deployment-bridge/issues/36).

Consider to implement a script (similar to scripts/getValidatorStartBlocks.js) that will get a private key from the environment variable (.env-file) and return an Ethereum address corresponding to the private key.

[fvictorio]

Done in a8c795d as part of #85.