Fix _install_branding

filter(provides=...) doesn't work with paths. The release packages
provide system-release so just look for that instead of a file.

Now it finds the release package and selects it along with the
corresponding logos package.

Note, this has been broken since commit 431ca6ce

src/pylorax/treebuilder.py

@@ -93,7 +93,8 @@ class RuntimeBuilder(object):
         release = None
         q = self.dbo.sack.query()
         a = q.available()
-        for pkg in a.filter(provides='/etc/system-release'):
+        for pkg in a.filter(provides='system-release'):
+            logger.debug("Found release package %s", pkg)
             if pkg.name.startswith('generic'):
                 continue
             else: