Should automatically pick up rotated instance credentials

[josefsachsconning]

If ec2_creds is used, then rotated credentials should automatically be picked up in order to support long running processes. In my experience, instance credentials tend to expire after about 6.5 hours, but new credentials are rotated in every hour or so.

[samoconnor]

Note that AWSCore.jl implements transparent update of instance credentials : https://github.com/samoconnor/AWSCore.jl/blob/master/src/AWSCore.jl#L154