Support IAM instance profile credentials

[copumpkin]

I'm trying to use awless from a machine that has valid (temporary) credentials on the standard 169.254.169.254 endpoint and it complains:

command hook failed: Your AWS credentials seem undefined!
AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY need to be exported in your CLI environment

My expectation from all other AWS tools and SDKs is that it would respect explicitly provided credentials if provided, but would fall back on the instance metadata credentials if those weren't provided.

[copumpkin]

A corollary to this would be to also optionally support a third environment variable for the session token provided by temporary credentials.

[copumpkin]

@jefe78 why the thumbs-down?

[fxaguessy]

We intentionally disabled this way to log into AWS, as it was slowing significantly the CLI, when no credentials were provided. It was freezing several minutes (trying to connect to 169.254.*) before throwing an error.
If we find a workaround to add this credential provider without slowing the CLI, we will put it back.

[copumpkin]

Weird, the standard AWS SDKs don't slow down at all; not sure what they do, but aws-sdk-go is one example in Go that I use all the time (in other tools) and I've never seen the freezing behavior.

[copumpkin]

It looks like they do this:

		cfg.HTTPClient = &http.Client{
			// use a shorter timeout than default because the metadata
			// service is local if it is running, and to fail faster
			// if not running on an ec2 instance.
			Timeout: 5 * time.Second,
		}

[fxaguessy]

This should be fixed in the last version of awless. Could you please test if it is working as expected ?