Remove duplicate code from acceptance tests.

Just cleaning things up here.
diff --git a/acceptance/02-list-servers.go b/acceptance/02-list-servers.go
index acfc802..64ec4f6 100644
--- a/acceptance/02-list-servers.go
+++ b/acceptance/02-list-servers.go
@@ -3,25 +3,13 @@
 import (
 	"fmt"
 	"github.com/rackspace/gophercloud"
-	"os"
 	"flag"
 )
 
 var quiet = flag.Bool("quiet", false, "Quiet mode, for acceptance testing.  $? still indicates errors though.")
 
 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()
 	flag.Parse()
 
 	acc, err := gophercloud.Authenticate(