Remove duplicate code from acceptance tests.

Just cleaning things up here.
diff --git a/acceptance/01-authentication.go b/acceptance/01-authentication.go
index 643ad48..91fc814 100644
--- a/acceptance/01-authentication.go
+++ b/acceptance/01-authentication.go
@@ -1,23 +1,11 @@
 package main
 
 import (
-	"os"
-	"fmt"
 	"github.com/rackspace/gophercloud"
 )
 
 func main() {
-	provider := os.Getenv("SDK_PROVIDER")
-	username := os.Getenv("SDK_USERNAME")
-	password := os.Getenv("SDK_PASSWORD")
-
-	if (provider == "") || (username == "") || (password == "") {
-		fmt.Fprintf(os.Stderr, "One or more of the following environment variables aren't set:\n")
-		fmt.Fprintf(os.Stderr, "  SDK_PROVIDER=\"%s\"\n", provider)
-		fmt.Fprintf(os.Stderr, "  SDK_USERNAME=\"%s\"\n", username)
-		fmt.Fprintf(os.Stderr, "  SDK_PASSWORD=\"%s\"\n", password)
-		os.Exit(1)
-	}
+	provider, username, password := getCredentials()
 
 	_, err := gophercloud.Authenticate(
 		provider,