Resolve slow fact gathering when seed SNAT is off

Stop configuring the seed host as gateway when seed SNAT is disabled
(which is the default since Xena), otherwise overcloud hosts may take a
long time to gather facts until their network configuration is applied
(probably because DNS servers are thought to be reachable until the
gateway is changed or removed).

Closes-Bug: #2039461
Change-Id: Ib847d3420dee374cec067cd8af519b510be04120