If signature available we must pop it from kw.

We should not send the signature in the property dict. Otherwise,
the validation of json schema will fail.

slapos/libnetworkcache.py

@@ -106,7 +106,7 @@ class NetworkcacheClient(object):
       if sha512 is None:
         kw['sha512'] = sha512sum
 
-      signature = kw.get('signature', None)
+      signature = kw.pop('signature', None)
       if signature is None:
         signature = ''
       data = [kw, signature]