Credentials loading via aws-sdk-go takes priority over goamz

 - The new library effectively brings following functionality:
   - load credentials from file at standard locations
   - use MFA session token with up-to-date naming convention

builtin/providers/aws/config.go

@@ -37,10 +37,17 @@ type AWSClient struct {
 // Client configures and returns a fully initailized AWSClient
 func (c *Config) Client() (interface{}, error) {
 	var client AWSClient
+	var errs []error
+
+	log.Println("[INFO] Detecting AWS creds via aws-sdk-go")
+	credsProvider := awsGo.DetectCreds(c.AccessKey, c.SecretKey, "")
+	_, err := credsProvider.Credentials()
+	if err != nil {
+		errs = append(errs, err)
+	}
 
 	// Get the auth and region. This can fail if keys/regions were not
 	// specified and we're attempting to use the environment.
-	var errs []error
 	log.Println("[INFO] Building AWS auth structure via goamz")
 	auth, err := c.AWSAuth()
 	if err != nil {
@@ -58,8 +65,6 @@ func (c *Config) Client() (interface{}, error) {
 		// bucket storage in S3
 		client.region = c.Region
 
-		credsProvider := awsGo.Creds(c.AccessKey, c.SecretKey, "")
-
 		log.Println("[INFO] Initializing EC2 connection")
 		client.ec2conn = ec2.New(auth, region)
 		log.Println("[INFO] Initializing ELB connection")