env_aws: Disable Retries and set Session cfg

Previously, Nomad used hand rolled HTTP requests to interact with the
EC2 metadata API. Recently however, we switched to using the AWS SDK for
this fingerprinting.

The default behaviour of the AWS SDK is to perform retries with
exponential backoff when a request fails. This is problematic for Nomad,
because interacting with the EC2 API is in our client start path.

Here we revert to our pre-existing behaviour of not performing retries
in the fast path, as if the metadata service is unavailable, it's likely
that nomad is not running in AWS.

client/fingerprint/env_aws.go

@@ -75,7 +75,10 @@ func (f *EnvAWSFingerprint) Fingerprint(request *FingerprintRequest, response *F
 		timeout = 1 * time.Millisecond
 	}
 
-	ec2meta := ec2MetaClient(f.endpoint, timeout)
+	ec2meta, err := ec2MetaClient(f.endpoint, timeout)
+	if err != nil {
+		return fmt.Errorf("failed to setup ec2 client: %v", err)
+	}
 
 	if !ec2meta.Available() {
 		return nil
@@ -191,15 +194,20 @@ func (f *EnvAWSFingerprint) linkSpeed(ec2meta *ec2metadata.EC2Metadata) int {
 	return netSpeed
 }
 
-func ec2MetaClient(endpoint string, timeout time.Duration) *ec2metadata.EC2Metadata {
+func ec2MetaClient(endpoint string, timeout time.Duration) (*ec2metadata.EC2Metadata, error) {
 	client := &http.Client{
 		Timeout:   timeout,
 		Transport: cleanhttp.DefaultTransport(),
 	}
 
-	c := aws.NewConfig().WithHTTPClient(client)
+	c := aws.NewConfig().WithHTTPClient(client).WithMaxRetries(0)
 	if endpoint != "" {
 		c = c.WithEndpoint(endpoint)
 	}
-	return ec2metadata.New(session.New(), c)
+
+	session, err := session.NewSession(c)
+	if err != nil {
+		return nil, err
+	}
+	return ec2metadata.New(session, c), nil
 }