Fix DHCP agent to work without extra_dhcp_opt extension

With the introduction of extra_dhcp_opt extension, DHCP agent
stopped serving mac adresses when used in conjunction with a
plugin that doesn't support this extension. This fixes the agent
by not assuming that port dict contains extra_dhcp_opt key.

Closes Bug: #1219893

Change-Id: I5bb52f3092f04c5e332b43cb3a04de05edc29cf5