ostbuild: Pull patches from specified patches git repository

We need to be more formal about where we get patches from, and more
specifically what version.  Let's assume they're also stored in a git
repository that we fetch, instead of copying them from "wherever the
manifest is".

This meshes nicely with splitting between manifest.json and
snapshot.json.

gnomeos/3.4/manifest.json

@@ -23,7 +23,8 @@
 		"netfilter": "git:git://git.netfilter.org/",
 		"cgwalters": "git:git://github.com/cgwalters/"},
 
-  "patches": {"src": "gnome:ostree"},
+  "patches": {"src": "gnome:ostree",
+              "prefix": "gnomeos/3.4"},
 
   "components": [
    		{"src": "cgwalters:ginitscripts"},