[aws] nixops doesnt support AWS_SESSION_TOKEN for MFA

[Ariakenom]

I was getting authorization errors trying to use nixops. Looking at the failing files locally I noticed that they did not account for the token, I think the failing files were from boto. I haven't checked too carefully but the some boto3 docs do mention the token while some boto docs don't.

[boto] http://docs.pythonboto.org/en/latest/getting_started.html
[boto3] https://boto3.amazonaws.com/v1/documentation/api/latest/guide/configuration.html

$ nixops --version
NixOps 1.7