[beta.2] eks does not work with assume-role

[abelanger5]

Assuming a role via AWS STS requires a session token that we do not query for.

Since we use NewStaticCredentials to initialize an API user, we can pass the session token in as well.

We should also support reading in from env variables and/or reading a local profile for a user. This would make setup much easier than manually passing in an id/secret/token.

[abelanger5]

If a local Porter instance is running (i.e. started with porter server start, which uses the local driver by default), this was fixed by #109

However, for external cluster connections from inside a cluster or Docker containers, we still need to support this.