dhcp: Default to using local DNS resolution

dhcp: Default to using local DNS resolution

It's pointless to not include default DNS resolution for Neutron.
This adds a new config option (dnsmasq_local_resolv) which defaults
to 'True' and will allow for DNS name resolution to work out of
the box. The caveat is that if the 'dnsmasq_dns_servers' is set it
will override the 'dnsmasq_local_resolv' setting, thus allowing
operators to explicitly set their own DNS servers.

DocImpact: Default to using local DNS resolution with the DHCP agent.

Change-Id: I17a884f467d307432a06f67a9dd93ed2fa6081a3
Closes-Bug: #1466117
Signed-off-by: Kyle Mestery <mestery@mestery.com>