ostree: use the correct build tags

Starting from 9b902d0, the ostree transport is disabled by default,
and ostree is enabled with the tag containers_image_ostree.

Signed-off-by: Marco Vedovati <mvedovati@suse.com>

hack/ostree_tag.sh

@@ -1,6 +1,4 @@
 #!/bin/bash
 if pkg-config ostree-1 2> /dev/null ; then
-	echo ostree
+	echo containers_image_ostree
-else
-	echo containers_image_ostree_stub
 fi