Delete DNS Name Server option if empty
Delete DNS Name Server option if empty

If Infoblox DNS Name Server option contain empty string, the
option need to be deleted. Otherwise, Infoblox will treat it as
an entry with a blank DNS server IP.

Change-Id: I36cff912249351aace463b2a57e3c09427014a2a

openstack/networking-infoblox,openstack/networking-infoblox