Avoid saving account 57 key in AWS lambda Environment Variables.

[anighanta]

Summary
Currently ERT uses the AWS Lambda Environment variables to read some data.
One of them is the account 57 private key for the network the ERT is targeting.
This is not entirely secure.

Possible resolution
We can use AWS KMS to store the key and KMS can then sign the fileUpdate transaction that ERT submits to the node.