Add option to load KMS master key ID from the CREDSTASH_KMS_KEY_ID environment variable

[alexmbird]

This PR adds the option to declare the KMS key using an environment variable. If the environment variable is unset, the original alias/credstash default will prevail.

The main use case of this is supporting the use of multiple credstashes in a single AWS account in an configure-and-forget manner.

Bonus: my text editor has automatically stripped trailing spaces from a bunch of lines :)

[jason-fugue]

Thanks for the PR! I think this is a good change, but it looks like the whitespace changes + another PR I merged earlier resulted in a conflict. Could you please resolve the conflict so that I can get this merged in?