It is possible that there are several public networks exist.
ExistingNeutronNetwork used to pick first one of them.
Now when internal network name or ID supplied it tries
to find such public network that the internal network has
a common router with.
Change-Id: I252bb5545891903355cf7f8480a2cd76febcfebe
Closes-Bug: #1524322
04bccb55b9