dhcp agent: make dnsmasq tags work with RHEL 6

rhbz 887369

RHEL 6 has dnsmasq-2.48 which has a different syntax for tags compared
with newer (>= dnsmasq-2.53). This patch updates command line and
options file contents to work with dnsmasq-2.48.

Change-Id: Ibf9309007fbf62f1f176737f2a5d179eabcf89dc
Signed-off-by: Chris Wright <chrisw@redhat.com>