Update infoblox-client requirement to 0.4.15
Update infoblox-client requirement to 0.4.15

Updated infoblox-client requirement from 0.4.14 to 0.4.15.
This version includes changes in logic for generating duid
using mac address.

Change-Id: Ic386f0e7908bf392caeaa47d371f9cdf6fd96f5b

openstack/networking-infoblox,openstack/networking-infoblox