Add extra_dhcp_opt extension to BigSwitch/Floodlight plugin

Adds support for the DHCP options extenion to the
BigSwitch/Floodlight plugin.

Fixes: bug #1206655
Change-Id: Ida30ca44ecbb9d1208303e47519427a4111f6025
Merge "Add extra_dhcp_opt extension to BigSwitch/Floodlight plugin"