Backport of https://review.openstack.org/#/c/108272/3 from Juno tip.

It has yet to be fully accepted, but I don't forsee any major code changes and this should address our issue with the dhcpnak replies after a restart. Pulled from: https://review.openstack.org/#/c/108272/

Fix dnsmasq lease lost upon restart

Replace option --leasefile-ro by --dhcp-leasefile for dnsmasq to
avoid losing lease information upon dhcp-agent restart.

Change-Id: I8230dc0ee30efef236b6827dd7116aba88df79de
Closes-Bug: 1345947
Partial-rally-bug: DE494