Support optional 4 arg for nova-dhcpbridge

dnsmasq will call the add, old, and del actions with an optional
4 argument of hostname if the hostname is known. Since we populate
the hostname in the DHCP host file its always passed to us.

Add an optional 4 argument for hostname for the add, old, and del
actions.

Change-Id: I1a6c095324be2424d6c1d469877b254f88fba4aa
Fixes: bug #1103783
Merge "Support optional 4 arg for nova-dhcpbridge"