Adding netmask to dnsmasq argument --dhcp-range

By default, dnsmasq will try to guess the proper subnet size but
sometimes it will fail. This patch provides the network netmask
to dnsmasq via the --dhcp-range argument.

Fixes: bug #1154929

Change-Id: I74d62c837b5310004b9151851697e3ea3b475f92
(cherry picked from commit fb151b5e3704c9a293f10ba89abc14f83d803c24)