Don't use duplicate IP addresses

Neutron is going to start rejecting attempts to create multiple members
in the same pool with duplicate IP address/port tuples.  This change
updates the tempest gate to make it stop trying to create new members
with the same IP address.

Change-Id: If8f88f2897728d6be63438847a1fa2320e56b1ae
Closes-Bug: #1252834
Merge "Don't use duplicate IP addresses"