utils: Fix untimeouted request in _ReadAsJson

This could cause slapos request to hang forever while fetching the
json schema of an SR, and in particular it could cause buildout to
hang forever in instance processing when requesting a sub-instance.

See merge request nexedi/slapos.core!575

slapos/util.py

@@ -424,7 +424,7 @@ def _readAsJson(url):
         return None
   if url.startswith('http://') or url.startswith('https://'):
     try:
-      r = requests.get(url)
+      r = requests.get(url, timeout=60) # we need a timeout !
       r.raise_for_status()
       return r.json()
     except (requests.exceptions.RequestException, ValueError):