Allow burnin to take "-t plain" as an option

The burnin code deals with "-t plain", but the command line parser
doesn't allow that as an option. This patch fixes this issue.

Reviewed-by: ultrotter

tools/burnin

@@ -122,7 +122,7 @@ class Burner(object):
                       help="Skip instance failovers", action="store_false",
                       default=True)
     parser.add_option("-t", "--disk-template", dest="disk_template",
-                      choices=("remote_raid1", "drbd"),
+                      choices=("plain", "remote_raid1", "drbd"),
                       default="remote_raid1",
                       help="Template type for network mirroring (remote_raid1"
                       " or drbd) [remote_raid1]")