Adds an option to enable broadcast replies to Dnsmasq

Adds a flag for DHCP agent configuration
to add dhcp-broadcast flag to Dnsmasq process
In order to support virtual network on top of Infiniband
Fabric, there is a requirement to receive DHCP response
via broadcast message (according to IB Spec).

DocImpact
Change-Id: I95329e535bacf14b650a8f37179ecd020b5bfafe
Closes-Bug: #1293953
Merge "Adds an option to enable broadcast replies to Dnsmasq"