restart dnsmasq when subnet cidr set changes

restart dnsmasq when subnet cidr set changes

fixes bug 1045828

The original bug occurred because the set of subnet cidrs changed
without a restart of dnsmasq.  As a result, dnsmasq would not respond
with offers for fixed_ips assigned in the unknown cidr ranges even when
the underlying host file had properly updated.

This patch addresses the bug by restarting dnsmasq when the set of
subnet cidrs changes.  When the set does not change, it will reload the
options and allocations.

Change-Id: I0e257208750703f4a32c51d1323786e76e676bb6