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")