Support dnsmasq_config_file option for dhcp-agent

Allow to configure dnsmasq_config_file for the dhcp-agent.
For common things like adapting the MTU size for instances
via DHCP.

closes-bug #1259495

Change-Id: Iee09eb9a66e84d49c0452fccf9c7d6bcb7ba8311
Merge "Support dnsmasq_config_file option for dhcp-agent"