Fix 04-create-server test to use a common prefix.

We need servers to dispose of for the 07-delete-server acceptance test.
Creating a bunch of servers in that test only to turn around and delete
them again will be unnecessarily time-consuming.  Instead, we'll use
ListServers() to discover a set of pre-existing servers to delete.
04-create-server already creates a batch, and will always execute before
07-delete-server (since 4 < 7).  Altering 04-create-server to put a tag
in the name of the servers it creates will allow 07-delete-server to
find them and delete them again.

The only exception is if any intervening tests fail.  You'll need to
clean up manually in that case.
diff --git a/acceptance/04-create-server.go b/acceptance/04-create-server.go
index bc5ff69..9d29b0e 100644
--- a/acceptance/04-create-server.go
+++ b/acceptance/04-create-server.go
@@ -15,7 +15,7 @@
 func configure() {
 	provider, username, password = getCredentials()
 	region = flag.String("r", "DFW", "Rackspace region in which to create the server")
-	serverName = flag.String("n", randomString(16), "Server name (what you see in the control panel)")
+	serverName = flag.String("n", randomString("ACPTTEST--", 16), "Server name (what you see in the control panel)")
 	imageRef = flag.String("i", "", "ID of image to deploy onto the server")
 	flavorRef = flag.String("f", "", "Flavor of server to deploy image upon")
 
diff --git a/acceptance/libargs.go b/acceptance/libargs.go
index 0af62d4..1802591 100644
--- a/acceptance/libargs.go
+++ b/acceptance/libargs.go
@@ -29,12 +29,12 @@
 // All content will be within the ASCII graphic character set.
 // (Implementation from Even Shaw's contribution on
 // http://stackoverflow.com/questions/12771930/what-is-the-fastest-way-to-generate-a-long-random-string-in-go).
-func randomString(n int) string {
+func randomString(prefix string, n int) string {
     const alphanum = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
     var bytes = make([]byte, n)
     rand.Read(bytes)
     for i, b := range bytes {
         bytes[i] = alphanum[b % byte(len(alphanum))]
     }
-    return string(bytes)
+    return prefix + string(bytes)
 }
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