Write correct upstream nameservers

Fuelmenu was saving DNS_UPSTREAM with the format
server1,server2 instead of server1 server2,
breaking DNS lookups.

Change-Id: I0c06454d867b995bc7d51df0a0199a21e1481384
Partial-Bug: #1327799
(cherry picked from commit 1512e723c76dd2abaf7ae834dedc14012864545d)