Fix Wrong syntax for set:tag in dnsmasq startup option

Looks like the introduction of quotes in fix for previous
bug 1012443 was erroneous.

Fixes LP# 1152504

Change-Id: Ic84739e38e3b3567f1617302a33623de03196f85
(cherry picked from commit d8188ef32293659c5a12d99f82ac19b452184967)